feat: add tenant + auth backend endpoints (Phase 1A)

Tenant management:
- POST /api/tenants — create tenant (creator becomes owner)
- GET /api/tenants — list tenants for authenticated user
- GET /api/tenants/:id — tenant details with access check
- POST /api/tenants/:id/invite — invite user by email (owner/admin)
- DELETE /api/tenants/:id/members/:uid — remove member
- GET /api/tenants/:id/members — list members

New packages:
- internal/services/tenant_service.go — CRUD on tenants + user_tenants
- internal/handlers/tenant_handler.go — HTTP handlers with auth checks
- internal/auth/tenant_resolver.go — X-Tenant-ID header middleware,
  defaults to user's first tenant for scoped routes

Authorization: owners/admins can invite and remove members. Cannot
remove the last owner. Users can remove themselves. TenantResolver
applies to resource routes (cases, deadlines, etc.) but not tenant
management routes.

backend/cmd/server/main.go

@@ -1,25 +1,32 @@
 package main
 
 import (
-	"fmt"
 	"log"
 	"net/http"
-	"os"
+
+	"mgit.msbls.de/m/KanzlAI-mGMT/internal/auth"
+	"mgit.msbls.de/m/KanzlAI-mGMT/internal/config"
+	"mgit.msbls.de/m/KanzlAI-mGMT/internal/db"
+	"mgit.msbls.de/m/KanzlAI-mGMT/internal/router"
 )
 
 func main() {
-	port := os.Getenv("PORT")
-	if port == "" {
-		port = "8080"
+	cfg, err := config.Load()
+	if err != nil {
+		log.Fatalf("Failed to load config: %v", err)
 	}
 
-	http.HandleFunc("/health", func(w http.ResponseWriter, r *http.Request) {
-		w.WriteHeader(http.StatusOK)
-		fmt.Fprintf(w, "ok")
-	})
+	database, err := db.Connect(cfg.DatabaseURL)
+	if err != nil {
+		log.Fatalf("Failed to connect to database: %v", err)
+	}
+	defer database.Close()
 
-	log.Printf("Starting KanzlAI API server on :%s", port)
-	if err := http.ListenAndServe(":"+port, nil); err != nil {
+	authMW := auth.NewMiddleware(cfg.SupabaseJWTSecret)
+	handler := router.New(database, authMW)
+
+	log.Printf("Starting KanzlAI API server on :%s", cfg.Port)
+	if err := http.ListenAndServe(":"+cfg.Port, handler); err != nil {
 		log.Fatal(err)
 	}
 }

backend/go.mod

@@ -1,3 +1,10 @@
 module mgit.msbls.de/m/KanzlAI-mGMT
 
 go 1.25.5
+
+require (
+	github.com/golang-jwt/jwt/v5 v5.3.1 // indirect
+	github.com/google/uuid v1.6.0 // indirect
+	github.com/jmoiron/sqlx v1.4.0 // indirect
+	github.com/lib/pq v1.12.0 // indirect
+)

backend/go.sum

@@ -0,0 +1,12 @@
+filippo.io/edwards25519 v1.1.0/go.mod h1:BxyFTGdWcka3PhytdK4V28tE5sGfRvvvRV7EaN4VDT4=
+github.com/go-sql-driver/mysql v1.8.1/go.mod h1:wEBSXgmK//2ZFJyE+qWnIsVGmvmEKlqwuVSjsCm7DZg=
+github.com/golang-jwt/jwt/v5 v5.3.1 h1:kYf81DTWFe7t+1VvL7eS+jKFVWaUnK9cB1qbwn63YCY=
+github.com/golang-jwt/jwt/v5 v5.3.1/go.mod h1:fxCRLWMO43lRc8nhHWY6LGqRcf+1gQWArsqaEUEa5bE=
+github.com/google/uuid v1.6.0 h1:NIvaJDMOsjHA8n1jAhLSgzrAzy1Hgr+hNrb57e+94F0=
+github.com/google/uuid v1.6.0/go.mod h1:TIyPZe4MgqvfeYDBFedMoGGpEw/LqOeaOT+nhxU+yHo=
+github.com/jmoiron/sqlx v1.4.0 h1:1PLqN7S1UYp5t4SrVVnt4nUVNemrDAtxlulVe+Qgm3o=
+github.com/jmoiron/sqlx v1.4.0/go.mod h1:ZrZ7UsYB/weZdl2Bxg6jCRO9c3YHl8r3ahlKmRT4JLY=
+github.com/lib/pq v1.10.9/go.mod h1:AlVN5x4E4T544tWzH6hKfbfQvm3HdbOxrmggDNAPY9o=
+github.com/lib/pq v1.12.0 h1:mC1zeiNamwKBecjHarAr26c/+d8V5w/u4J0I/yASbJo=
+github.com/lib/pq v1.12.0/go.mod h1:/p+8NSbOcwzAEI7wiMXFlgydTwcgTr3OSKMsD2BitpA=
+github.com/mattn/go-sqlite3 v1.14.22/go.mod h1:Uh1q+B4BYcTPb+yiD3kU8Ct7aC0hY9fxUwlHK0RXw+Y=

backend/internal/auth/context.go

@@ -0,0 +1,32 @@
+package auth
+
+import (
+	"context"
+
+	"github.com/google/uuid"
+)
+
+type contextKey string
+
+const (
+	userIDKey   contextKey = "user_id"
+	tenantIDKey contextKey = "tenant_id"
+)
+
+func ContextWithUserID(ctx context.Context, userID uuid.UUID) context.Context {
+	return context.WithValue(ctx, userIDKey, userID)
+}
+
+func ContextWithTenantID(ctx context.Context, tenantID uuid.UUID) context.Context {
+	return context.WithValue(ctx, tenantIDKey, tenantID)
+}
+
+func UserFromContext(ctx context.Context) (uuid.UUID, bool) {
+	id, ok := ctx.Value(userIDKey).(uuid.UUID)
+	return id, ok
+}
+
+func TenantFromContext(ctx context.Context) (uuid.UUID, bool) {
+	id, ok := ctx.Value(tenantIDKey).(uuid.UUID)
+	return id, ok
+}

backend/internal/auth/middleware.go

@@ -0,0 +1,89 @@
+package auth
+
+import (
+	"fmt"
+	"net/http"
+	"strings"
+	"time"
+
+	"github.com/golang-jwt/jwt/v5"
+	"github.com/google/uuid"
+)
+
+type Middleware struct {
+	jwtSecret []byte
+}
+
+func NewMiddleware(jwtSecret string) *Middleware {
+	return &Middleware{jwtSecret: []byte(jwtSecret)}
+}
+
+func (m *Middleware) RequireAuth(next http.Handler) http.Handler {
+	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		token := extractBearerToken(r)
+		if token == "" {
+			http.Error(w, "missing authorization token", http.StatusUnauthorized)
+			return
+		}
+
+		userID, err := m.verifyJWT(token)
+		if err != nil {
+			http.Error(w, fmt.Sprintf("invalid token: %v", err), http.StatusUnauthorized)
+			return
+		}
+
+		ctx := ContextWithUserID(r.Context(), userID)
+		next.ServeHTTP(w, r.WithContext(ctx))
+	})
+}
+
+func (m *Middleware) verifyJWT(tokenStr string) (uuid.UUID, error) {
+	parsedToken, err := jwt.Parse(tokenStr, func(t *jwt.Token) (interface{}, error) {
+		if _, ok := t.Method.(*jwt.SigningMethodHMAC); !ok {
+			return nil, fmt.Errorf("unexpected signing method: %v", t.Header["alg"])
+		}
+		return m.jwtSecret, nil
+	})
+	if err != nil {
+		return uuid.Nil, fmt.Errorf("parsing JWT: %w", err)
+	}
+
+	if !parsedToken.Valid {
+		return uuid.Nil, fmt.Errorf("invalid JWT token")
+	}
+
+	claims, ok := parsedToken.Claims.(jwt.MapClaims)
+	if !ok {
+		return uuid.Nil, fmt.Errorf("extracting JWT claims")
+	}
+
+	if exp, ok := claims["exp"].(float64); ok {
+		if time.Now().Unix() > int64(exp) {
+			return uuid.Nil, fmt.Errorf("JWT token has expired")
+		}
+	}
+
+	sub, ok := claims["sub"].(string)
+	if !ok {
+		return uuid.Nil, fmt.Errorf("missing sub claim in JWT")
+	}
+
+	userID, err := uuid.Parse(sub)
+	if err != nil {
+		return uuid.Nil, fmt.Errorf("invalid user ID format: %w", err)
+	}
+
+	return userID, nil
+}
+
+func extractBearerToken(r *http.Request) string {
+	auth := r.Header.Get("Authorization")
+	if auth == "" {
+		return ""
+	}
+	parts := strings.SplitN(auth, " ", 2)
+	if len(parts) != 2 || !strings.EqualFold(parts[0], "bearer") {
+		return ""
+	}
+	return parts[1]
+}

backend/internal/auth/tenant_resolver.go

@@ -0,0 +1,61 @@
+package auth
+
+import (
+	"context"
+	"fmt"
+	"net/http"
+
+	"github.com/google/uuid"
+)
+
+// TenantLookup resolves the default tenant for a user.
+// Defined as an interface to avoid circular dependency with services.
+type TenantLookup interface {
+	FirstTenantForUser(ctx context.Context, userID uuid.UUID) (*uuid.UUID, error)
+}
+
+// TenantResolver is middleware that resolves the tenant from X-Tenant-ID header
+// or defaults to the user's first tenant.
+type TenantResolver struct {
+	lookup TenantLookup
+}
+
+func NewTenantResolver(lookup TenantLookup) *TenantResolver {
+	return &TenantResolver{lookup: lookup}
+}
+
+func (tr *TenantResolver) Resolve(next http.Handler) http.Handler {
+	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		userID, ok := UserFromContext(r.Context())
+		if !ok {
+			http.Error(w, "unauthorized", http.StatusUnauthorized)
+			return
+		}
+
+		var tenantID uuid.UUID
+
+		if header := r.Header.Get("X-Tenant-ID"); header != "" {
+			parsed, err := uuid.Parse(header)
+			if err != nil {
+				http.Error(w, fmt.Sprintf("invalid X-Tenant-ID: %v", err), http.StatusBadRequest)
+				return
+			}
+			tenantID = parsed
+		} else {
+			// Default to user's first tenant
+			first, err := tr.lookup.FirstTenantForUser(r.Context(), userID)
+			if err != nil {
+				http.Error(w, fmt.Sprintf("resolving tenant: %v", err), http.StatusInternalServerError)
+				return
+			}
+			if first == nil {
+				http.Error(w, "no tenant found for user", http.StatusBadRequest)
+				return
+			}
+			tenantID = *first
+		}
+
+		ctx := ContextWithTenantID(r.Context(), tenantID)
+		next.ServeHTTP(w, r.WithContext(ctx))
+	})
+}

backend/internal/auth/tenant_resolver_test.go

@@ -0,0 +1,124 @@
+package auth
+
+import (
+	"context"
+	"net/http"
+	"net/http/httptest"
+	"testing"
+
+	"github.com/google/uuid"
+)
+
+type mockTenantLookup struct {
+	tenantID *uuid.UUID
+	err      error
+}
+
+func (m *mockTenantLookup) FirstTenantForUser(ctx context.Context, userID uuid.UUID) (*uuid.UUID, error) {
+	return m.tenantID, m.err
+}
+
+func TestTenantResolver_FromHeader(t *testing.T) {
+	tenantID := uuid.New()
+	tr := NewTenantResolver(&mockTenantLookup{})
+
+	var gotTenantID uuid.UUID
+	next := http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		id, ok := TenantFromContext(r.Context())
+		if !ok {
+			t.Fatal("tenant ID not in context")
+		}
+		gotTenantID = id
+		w.WriteHeader(http.StatusOK)
+	})
+
+	r := httptest.NewRequest("GET", "/api/cases", nil)
+	r.Header.Set("X-Tenant-ID", tenantID.String())
+	r = r.WithContext(ContextWithUserID(r.Context(), uuid.New()))
+	w := httptest.NewRecorder()
+
+	tr.Resolve(next).ServeHTTP(w, r)
+
+	if w.Code != http.StatusOK {
+		t.Fatalf("expected 200, got %d", w.Code)
+	}
+	if gotTenantID != tenantID {
+		t.Errorf("expected tenant %s, got %s", tenantID, gotTenantID)
+	}
+}
+
+func TestTenantResolver_DefaultsToFirst(t *testing.T) {
+	tenantID := uuid.New()
+	tr := NewTenantResolver(&mockTenantLookup{tenantID: &tenantID})
+
+	var gotTenantID uuid.UUID
+	next := http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		id, _ := TenantFromContext(r.Context())
+		gotTenantID = id
+		w.WriteHeader(http.StatusOK)
+	})
+
+	r := httptest.NewRequest("GET", "/api/cases", nil)
+	r = r.WithContext(ContextWithUserID(r.Context(), uuid.New()))
+	w := httptest.NewRecorder()
+
+	tr.Resolve(next).ServeHTTP(w, r)
+
+	if w.Code != http.StatusOK {
+		t.Fatalf("expected 200, got %d", w.Code)
+	}
+	if gotTenantID != tenantID {
+		t.Errorf("expected tenant %s, got %s", tenantID, gotTenantID)
+	}
+}
+
+func TestTenantResolver_NoUser(t *testing.T) {
+	tr := NewTenantResolver(&mockTenantLookup{})
+	next := http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		t.Fatal("next should not be called")
+	})
+
+	r := httptest.NewRequest("GET", "/api/cases", nil)
+	w := httptest.NewRecorder()
+
+	tr.Resolve(next).ServeHTTP(w, r)
+
+	if w.Code != http.StatusUnauthorized {
+		t.Errorf("expected 401, got %d", w.Code)
+	}
+}
+
+func TestTenantResolver_InvalidHeader(t *testing.T) {
+	tr := NewTenantResolver(&mockTenantLookup{})
+	next := http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		t.Fatal("next should not be called")
+	})
+
+	r := httptest.NewRequest("GET", "/api/cases", nil)
+	r.Header.Set("X-Tenant-ID", "not-a-uuid")
+	r = r.WithContext(ContextWithUserID(r.Context(), uuid.New()))
+	w := httptest.NewRecorder()
+
+	tr.Resolve(next).ServeHTTP(w, r)
+
+	if w.Code != http.StatusBadRequest {
+		t.Errorf("expected 400, got %d", w.Code)
+	}
+}
+
+func TestTenantResolver_NoTenantForUser(t *testing.T) {
+	tr := NewTenantResolver(&mockTenantLookup{tenantID: nil})
+	next := http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		t.Fatal("next should not be called")
+	})
+
+	r := httptest.NewRequest("GET", "/api/cases", nil)
+	r = r.WithContext(ContextWithUserID(r.Context(), uuid.New()))
+	w := httptest.NewRecorder()
+
+	tr.Resolve(next).ServeHTTP(w, r)
+
+	if w.Code != http.StatusBadRequest {
+		t.Errorf("expected 400, got %d", w.Code)
+	}
+}

backend/internal/config/config.go

@@ -0,0 +1,42 @@
+package config
+
+import (
+	"fmt"
+	"os"
+)
+
+type Config struct {
+	Port             string
+	DatabaseURL      string
+	SupabaseURL      string
+	SupabaseAnonKey  string
+	SupabaseJWTSecret string
+	AnthropicAPIKey  string
+}
+
+func Load() (*Config, error) {
+	cfg := &Config{
+		Port:             getEnv("PORT", "8080"),
+		DatabaseURL:      os.Getenv("DATABASE_URL"),
+		SupabaseURL:      os.Getenv("SUPABASE_URL"),
+		SupabaseAnonKey:  os.Getenv("SUPABASE_ANON_KEY"),
+		SupabaseJWTSecret: os.Getenv("SUPABASE_JWT_SECRET"),
+		AnthropicAPIKey:  os.Getenv("ANTHROPIC_API_KEY"),
+	}
+
+	if cfg.DatabaseURL == "" {
+		return nil, fmt.Errorf("DATABASE_URL is required")
+	}
+	if cfg.SupabaseJWTSecret == "" {
+		return nil, fmt.Errorf("SUPABASE_JWT_SECRET is required")
+	}
+
+	return cfg, nil
+}
+
+func getEnv(key, fallback string) string {
+	if v := os.Getenv(key); v != "" {
+		return v
+	}
+	return fallback
+}

backend/internal/db/connection.go

@@ -0,0 +1,26 @@
+package db
+
+import (
+	"fmt"
+
+	"github.com/jmoiron/sqlx"
+	_ "github.com/lib/pq"
+)
+
+func Connect(databaseURL string) (*sqlx.DB, error) {
+	db, err := sqlx.Connect("postgres", databaseURL)
+	if err != nil {
+		return nil, fmt.Errorf("connecting to database: %w", err)
+	}
+
+	// Set search_path so queries use kanzlai schema by default
+	if _, err := db.Exec("SET search_path TO kanzlai, public"); err != nil {
+		db.Close()
+		return nil, fmt.Errorf("setting search_path: %w", err)
+	}
+
+	db.SetMaxOpenConns(25)
+	db.SetMaxIdleConns(5)
+
+	return db, nil
+}

backend/internal/handlers/tenant_handler.go

@@ -0,0 +1,243 @@
+package handlers
+
+import (
+	"encoding/json"
+	"net/http"
+
+	"github.com/google/uuid"
+
+	"mgit.msbls.de/m/KanzlAI-mGMT/internal/auth"
+	"mgit.msbls.de/m/KanzlAI-mGMT/internal/services"
+)
+
+type TenantHandler struct {
+	svc *services.TenantService
+}
+
+func NewTenantHandler(svc *services.TenantService) *TenantHandler {
+	return &TenantHandler{svc: svc}
+}
+
+// CreateTenant handles POST /api/tenants
+func (h *TenantHandler) CreateTenant(w http.ResponseWriter, r *http.Request) {
+	userID, ok := auth.UserFromContext(r.Context())
+	if !ok {
+		http.Error(w, "unauthorized", http.StatusUnauthorized)
+		return
+	}
+
+	var req struct {
+		Name string `json:"name"`
+		Slug string `json:"slug"`
+	}
+	if err := json.NewDecoder(r.Body).Decode(&req); err != nil {
+		jsonError(w, "invalid request body", http.StatusBadRequest)
+		return
+	}
+	if req.Name == "" || req.Slug == "" {
+		jsonError(w, "name and slug are required", http.StatusBadRequest)
+		return
+	}
+
+	tenant, err := h.svc.Create(r.Context(), userID, req.Name, req.Slug)
+	if err != nil {
+		jsonError(w, err.Error(), http.StatusInternalServerError)
+		return
+	}
+
+	jsonResponse(w, tenant, http.StatusCreated)
+}
+
+// ListTenants handles GET /api/tenants
+func (h *TenantHandler) ListTenants(w http.ResponseWriter, r *http.Request) {
+	userID, ok := auth.UserFromContext(r.Context())
+	if !ok {
+		http.Error(w, "unauthorized", http.StatusUnauthorized)
+		return
+	}
+
+	tenants, err := h.svc.ListForUser(r.Context(), userID)
+	if err != nil {
+		jsonError(w, err.Error(), http.StatusInternalServerError)
+		return
+	}
+
+	jsonResponse(w, tenants, http.StatusOK)
+}
+
+// GetTenant handles GET /api/tenants/{id}
+func (h *TenantHandler) GetTenant(w http.ResponseWriter, r *http.Request) {
+	userID, ok := auth.UserFromContext(r.Context())
+	if !ok {
+		http.Error(w, "unauthorized", http.StatusUnauthorized)
+		return
+	}
+
+	tenantID, err := uuid.Parse(r.PathValue("id"))
+	if err != nil {
+		jsonError(w, "invalid tenant ID", http.StatusBadRequest)
+		return
+	}
+
+	// Verify user has access to this tenant
+	role, err := h.svc.GetUserRole(r.Context(), userID, tenantID)
+	if err != nil {
+		jsonError(w, err.Error(), http.StatusInternalServerError)
+		return
+	}
+	if role == "" {
+		jsonError(w, "not found", http.StatusNotFound)
+		return
+	}
+
+	tenant, err := h.svc.GetByID(r.Context(), tenantID)
+	if err != nil {
+		jsonError(w, err.Error(), http.StatusInternalServerError)
+		return
+	}
+	if tenant == nil {
+		jsonError(w, "not found", http.StatusNotFound)
+		return
+	}
+
+	jsonResponse(w, tenant, http.StatusOK)
+}
+
+// InviteUser handles POST /api/tenants/{id}/invite
+func (h *TenantHandler) InviteUser(w http.ResponseWriter, r *http.Request) {
+	userID, ok := auth.UserFromContext(r.Context())
+	if !ok {
+		http.Error(w, "unauthorized", http.StatusUnauthorized)
+		return
+	}
+
+	tenantID, err := uuid.Parse(r.PathValue("id"))
+	if err != nil {
+		jsonError(w, "invalid tenant ID", http.StatusBadRequest)
+		return
+	}
+
+	// Only owners and admins can invite
+	role, err := h.svc.GetUserRole(r.Context(), userID, tenantID)
+	if err != nil {
+		jsonError(w, err.Error(), http.StatusInternalServerError)
+		return
+	}
+	if role != "owner" && role != "admin" {
+		jsonError(w, "only owners and admins can invite users", http.StatusForbidden)
+		return
+	}
+
+	var req struct {
+		Email string `json:"email"`
+		Role  string `json:"role"`
+	}
+	if err := json.NewDecoder(r.Body).Decode(&req); err != nil {
+		jsonError(w, "invalid request body", http.StatusBadRequest)
+		return
+	}
+	if req.Email == "" {
+		jsonError(w, "email is required", http.StatusBadRequest)
+		return
+	}
+	if req.Role == "" {
+		req.Role = "member"
+	}
+	if req.Role != "member" && req.Role != "admin" {
+		jsonError(w, "role must be member or admin", http.StatusBadRequest)
+		return
+	}
+
+	ut, err := h.svc.InviteByEmail(r.Context(), tenantID, req.Email, req.Role)
+	if err != nil {
+		jsonError(w, err.Error(), http.StatusBadRequest)
+		return
+	}
+
+	jsonResponse(w, ut, http.StatusCreated)
+}
+
+// RemoveMember handles DELETE /api/tenants/{id}/members/{uid}
+func (h *TenantHandler) RemoveMember(w http.ResponseWriter, r *http.Request) {
+	userID, ok := auth.UserFromContext(r.Context())
+	if !ok {
+		http.Error(w, "unauthorized", http.StatusUnauthorized)
+		return
+	}
+
+	tenantID, err := uuid.Parse(r.PathValue("id"))
+	if err != nil {
+		jsonError(w, "invalid tenant ID", http.StatusBadRequest)
+		return
+	}
+
+	memberID, err := uuid.Parse(r.PathValue("uid"))
+	if err != nil {
+		jsonError(w, "invalid member ID", http.StatusBadRequest)
+		return
+	}
+
+	// Only owners and admins can remove members (or user removing themselves)
+	role, err := h.svc.GetUserRole(r.Context(), userID, tenantID)
+	if err != nil {
+		jsonError(w, err.Error(), http.StatusInternalServerError)
+		return
+	}
+	if role != "owner" && role != "admin" && userID != memberID {
+		jsonError(w, "insufficient permissions", http.StatusForbidden)
+		return
+	}
+
+	if err := h.svc.RemoveMember(r.Context(), tenantID, memberID); err != nil {
+		jsonError(w, err.Error(), http.StatusBadRequest)
+		return
+	}
+
+	jsonResponse(w, map[string]string{"status": "removed"}, http.StatusOK)
+}
+
+// ListMembers handles GET /api/tenants/{id}/members
+func (h *TenantHandler) ListMembers(w http.ResponseWriter, r *http.Request) {
+	userID, ok := auth.UserFromContext(r.Context())
+	if !ok {
+		http.Error(w, "unauthorized", http.StatusUnauthorized)
+		return
+	}
+
+	tenantID, err := uuid.Parse(r.PathValue("id"))
+	if err != nil {
+		jsonError(w, "invalid tenant ID", http.StatusBadRequest)
+		return
+	}
+
+	// Verify user has access
+	role, err := h.svc.GetUserRole(r.Context(), userID, tenantID)
+	if err != nil {
+		jsonError(w, err.Error(), http.StatusInternalServerError)
+		return
+	}
+	if role == "" {
+		jsonError(w, "not found", http.StatusNotFound)
+		return
+	}
+
+	members, err := h.svc.ListMembers(r.Context(), tenantID)
+	if err != nil {
+		jsonError(w, err.Error(), http.StatusInternalServerError)
+		return
+	}
+
+	jsonResponse(w, members, http.StatusOK)
+}
+
+func jsonResponse(w http.ResponseWriter, data interface{}, status int) {
+	w.Header().Set("Content-Type", "application/json")
+	w.WriteHeader(status)
+	json.NewEncoder(w).Encode(data)
+}
+
+func jsonError(w http.ResponseWriter, msg string, status int) {
+	w.Header().Set("Content-Type", "application/json")
+	w.WriteHeader(status)
+	json.NewEncoder(w).Encode(map[string]string{"error": msg})
+}

backend/internal/handlers/tenant_handler_test.go

@@ -0,0 +1,132 @@
+package handlers
+
+import (
+	"bytes"
+	"encoding/json"
+	"net/http"
+	"net/http/httptest"
+	"testing"
+
+	"github.com/google/uuid"
+
+	"mgit.msbls.de/m/KanzlAI-mGMT/internal/auth"
+)
+
+func TestCreateTenant_MissingFields(t *testing.T) {
+	h := &TenantHandler{} // no service needed for validation
+
+	// Build request with auth context
+	body := `{"name":"","slug":""}`
+	r := httptest.NewRequest("POST", "/api/tenants", bytes.NewBufferString(body))
+	r = r.WithContext(auth.ContextWithUserID(r.Context(), uuid.New()))
+	w := httptest.NewRecorder()
+
+	h.CreateTenant(w, r)
+
+	if w.Code != http.StatusBadRequest {
+		t.Errorf("expected 400, got %d", w.Code)
+	}
+
+	var resp map[string]string
+	json.NewDecoder(w.Body).Decode(&resp)
+	if resp["error"] != "name and slug are required" {
+		t.Errorf("unexpected error: %s", resp["error"])
+	}
+}
+
+func TestCreateTenant_NoAuth(t *testing.T) {
+	h := &TenantHandler{}
+	r := httptest.NewRequest("POST", "/api/tenants", bytes.NewBufferString(`{}`))
+	w := httptest.NewRecorder()
+
+	h.CreateTenant(w, r)
+
+	if w.Code != http.StatusUnauthorized {
+		t.Errorf("expected 401, got %d", w.Code)
+	}
+}
+
+func TestGetTenant_InvalidID(t *testing.T) {
+	h := &TenantHandler{}
+	r := httptest.NewRequest("GET", "/api/tenants/not-a-uuid", nil)
+	r.SetPathValue("id", "not-a-uuid")
+	r = r.WithContext(auth.ContextWithUserID(r.Context(), uuid.New()))
+	w := httptest.NewRecorder()
+
+	h.GetTenant(w, r)
+
+	if w.Code != http.StatusBadRequest {
+		t.Errorf("expected 400, got %d", w.Code)
+	}
+}
+
+func TestInviteUser_InvalidTenantID(t *testing.T) {
+	h := &TenantHandler{}
+	body := `{"email":"test@example.com","role":"member"}`
+	r := httptest.NewRequest("POST", "/api/tenants/bad/invite", bytes.NewBufferString(body))
+	r.SetPathValue("id", "bad")
+	r = r.WithContext(auth.ContextWithUserID(r.Context(), uuid.New()))
+	w := httptest.NewRecorder()
+
+	h.InviteUser(w, r)
+
+	if w.Code != http.StatusBadRequest {
+		t.Errorf("expected 400, got %d", w.Code)
+	}
+}
+
+func TestInviteUser_NoAuth(t *testing.T) {
+	h := &TenantHandler{}
+	body := `{"email":"test@example.com"}`
+	r := httptest.NewRequest("POST", "/api/tenants/"+uuid.New().String()+"/invite", bytes.NewBufferString(body))
+	r.SetPathValue("id", uuid.New().String())
+	w := httptest.NewRecorder()
+
+	h.InviteUser(w, r)
+
+	if w.Code != http.StatusUnauthorized {
+		t.Errorf("expected 401, got %d", w.Code)
+	}
+}
+
+func TestRemoveMember_InvalidIDs(t *testing.T) {
+	h := &TenantHandler{}
+	r := httptest.NewRequest("DELETE", "/api/tenants/bad/members/bad", nil)
+	r.SetPathValue("id", "bad")
+	r.SetPathValue("uid", "bad")
+	r = r.WithContext(auth.ContextWithUserID(r.Context(), uuid.New()))
+	w := httptest.NewRecorder()
+
+	h.RemoveMember(w, r)
+
+	if w.Code != http.StatusBadRequest {
+		t.Errorf("expected 400, got %d", w.Code)
+	}
+}
+
+func TestJsonResponse(t *testing.T) {
+	w := httptest.NewRecorder()
+	jsonResponse(w, map[string]string{"key": "value"}, http.StatusOK)
+
+	if w.Code != http.StatusOK {
+		t.Errorf("expected 200, got %d", w.Code)
+	}
+	if ct := w.Header().Get("Content-Type"); ct != "application/json" {
+		t.Errorf("expected application/json, got %s", ct)
+	}
+}
+
+func TestJsonError(t *testing.T) {
+	w := httptest.NewRecorder()
+	jsonError(w, "something went wrong", http.StatusBadRequest)
+
+	if w.Code != http.StatusBadRequest {
+		t.Errorf("expected 400, got %d", w.Code)
+	}
+
+	var resp map[string]string
+	json.NewDecoder(w.Body).Decode(&resp)
+	if resp["error"] != "something went wrong" {
+		t.Errorf("unexpected error: %s", resp["error"])
+	}
+}

backend/internal/models/appointment.go

@@ -0,0 +1,23 @@
+package models
+
+import (
+	"time"
+
+	"github.com/google/uuid"
+)
+
+type Appointment struct {
+	ID              uuid.UUID  `db:"id" json:"id"`
+	TenantID        uuid.UUID  `db:"tenant_id" json:"tenant_id"`
+	CaseID          *uuid.UUID `db:"case_id" json:"case_id,omitempty"`
+	Title           string     `db:"title" json:"title"`
+	Description     *string    `db:"description" json:"description,omitempty"`
+	StartAt         time.Time  `db:"start_at" json:"start_at"`
+	EndAt           *time.Time `db:"end_at" json:"end_at,omitempty"`
+	Location        *string    `db:"location" json:"location,omitempty"`
+	AppointmentType *string    `db:"appointment_type" json:"appointment_type,omitempty"`
+	CalDAVUID       *string    `db:"caldav_uid" json:"caldav_uid,omitempty"`
+	CalDAVEtag      *string    `db:"caldav_etag" json:"caldav_etag,omitempty"`
+	CreatedAt       time.Time  `db:"created_at" json:"created_at"`
+	UpdatedAt       time.Time  `db:"updated_at" json:"updated_at"`
+}

backend/internal/models/case.go

@@ -0,0 +1,23 @@
+package models
+
+import (
+	"encoding/json"
+	"time"
+
+	"github.com/google/uuid"
+)
+
+type Case struct {
+	ID         uuid.UUID       `db:"id" json:"id"`
+	TenantID   uuid.UUID       `db:"tenant_id" json:"tenant_id"`
+	CaseNumber string          `db:"case_number" json:"case_number"`
+	Title      string          `db:"title" json:"title"`
+	CaseType   *string         `db:"case_type" json:"case_type,omitempty"`
+	Court      *string         `db:"court" json:"court,omitempty"`
+	CourtRef   *string         `db:"court_ref" json:"court_ref,omitempty"`
+	Status     string          `db:"status" json:"status"`
+	AISummary  *string         `db:"ai_summary" json:"ai_summary,omitempty"`
+	Metadata   json.RawMessage `db:"metadata" json:"metadata"`
+	CreatedAt  time.Time       `db:"created_at" json:"created_at"`
+	UpdatedAt  time.Time       `db:"updated_at" json:"updated_at"`
+}

backend/internal/models/case_event.go

@@ -0,0 +1,22 @@
+package models
+
+import (
+	"encoding/json"
+	"time"
+
+	"github.com/google/uuid"
+)
+
+type CaseEvent struct {
+	ID          uuid.UUID       `db:"id" json:"id"`
+	TenantID    uuid.UUID       `db:"tenant_id" json:"tenant_id"`
+	CaseID      uuid.UUID       `db:"case_id" json:"case_id"`
+	EventType   *string         `db:"event_type" json:"event_type,omitempty"`
+	Title       string          `db:"title" json:"title"`
+	Description *string         `db:"description" json:"description,omitempty"`
+	EventDate   *time.Time      `db:"event_date" json:"event_date,omitempty"`
+	CreatedBy   *uuid.UUID      `db:"created_by" json:"created_by,omitempty"`
+	Metadata    json.RawMessage `db:"metadata" json:"metadata"`
+	CreatedAt   time.Time       `db:"created_at" json:"created_at"`
+	UpdatedAt   time.Time       `db:"updated_at" json:"updated_at"`
+}

backend/internal/models/deadline.go

@@ -0,0 +1,27 @@
+package models
+
+import (
+	"time"
+
+	"github.com/google/uuid"
+)
+
+type Deadline struct {
+	ID              uuid.UUID  `db:"id" json:"id"`
+	TenantID        uuid.UUID  `db:"tenant_id" json:"tenant_id"`
+	CaseID          uuid.UUID  `db:"case_id" json:"case_id"`
+	Title           string     `db:"title" json:"title"`
+	Description     *string    `db:"description" json:"description,omitempty"`
+	DueDate         string     `db:"due_date" json:"due_date"`
+	OriginalDueDate *string    `db:"original_due_date" json:"original_due_date,omitempty"`
+	WarningDate     *string    `db:"warning_date" json:"warning_date,omitempty"`
+	Source          string     `db:"source" json:"source"`
+	RuleID          *uuid.UUID `db:"rule_id" json:"rule_id,omitempty"`
+	Status          string     `db:"status" json:"status"`
+	CompletedAt     *time.Time `db:"completed_at" json:"completed_at,omitempty"`
+	CalDAVUID       *string    `db:"caldav_uid" json:"caldav_uid,omitempty"`
+	CalDAVEtag      *string    `db:"caldav_etag" json:"caldav_etag,omitempty"`
+	Notes           *string    `db:"notes" json:"notes,omitempty"`
+	CreatedAt       time.Time  `db:"created_at" json:"created_at"`
+	UpdatedAt       time.Time  `db:"updated_at" json:"updated_at"`
+}

backend/internal/models/deadline_rule.go

@@ -0,0 +1,43 @@
+package models
+
+import (
+	"time"
+
+	"github.com/google/uuid"
+)
+
+type DeadlineRule struct {
+	ID               uuid.UUID  `db:"id" json:"id"`
+	ProceedingTypeID *int       `db:"proceeding_type_id" json:"proceeding_type_id,omitempty"`
+	ParentID         *uuid.UUID `db:"parent_id" json:"parent_id,omitempty"`
+	Code             *string    `db:"code" json:"code,omitempty"`
+	Name             string     `db:"name" json:"name"`
+	Description      *string    `db:"description" json:"description,omitempty"`
+	PrimaryParty     *string    `db:"primary_party" json:"primary_party,omitempty"`
+	EventType        *string    `db:"event_type" json:"event_type,omitempty"`
+	IsMandatory      bool       `db:"is_mandatory" json:"is_mandatory"`
+	DurationValue    int        `db:"duration_value" json:"duration_value"`
+	DurationUnit     string     `db:"duration_unit" json:"duration_unit"`
+	Timing           *string    `db:"timing" json:"timing,omitempty"`
+	RuleCode         *string    `db:"rule_code" json:"rule_code,omitempty"`
+	DeadlineNotes    *string    `db:"deadline_notes" json:"deadline_notes,omitempty"`
+	SequenceOrder    int        `db:"sequence_order" json:"sequence_order"`
+	ConditionRuleID  *uuid.UUID `db:"condition_rule_id" json:"condition_rule_id,omitempty"`
+	AltDurationValue *int       `db:"alt_duration_value" json:"alt_duration_value,omitempty"`
+	AltDurationUnit  *string    `db:"alt_duration_unit" json:"alt_duration_unit,omitempty"`
+	AltRuleCode      *string    `db:"alt_rule_code" json:"alt_rule_code,omitempty"`
+	IsActive         bool       `db:"is_active" json:"is_active"`
+	CreatedAt        time.Time  `db:"created_at" json:"created_at"`
+	UpdatedAt        time.Time  `db:"updated_at" json:"updated_at"`
+}
+
+type ProceedingType struct {
+	ID           int     `db:"id" json:"id"`
+	Code         string  `db:"code" json:"code"`
+	Name         string  `db:"name" json:"name"`
+	Description  *string `db:"description" json:"description,omitempty"`
+	Jurisdiction *string `db:"jurisdiction" json:"jurisdiction,omitempty"`
+	DefaultColor string  `db:"default_color" json:"default_color"`
+	SortOrder    int     `db:"sort_order" json:"sort_order"`
+	IsActive     bool    `db:"is_active" json:"is_active"`
+}

backend/internal/models/document.go

@@ -0,0 +1,23 @@
+package models
+
+import (
+	"encoding/json"
+	"time"
+
+	"github.com/google/uuid"
+)
+
+type Document struct {
+	ID          uuid.UUID        `db:"id" json:"id"`
+	TenantID    uuid.UUID        `db:"tenant_id" json:"tenant_id"`
+	CaseID      uuid.UUID        `db:"case_id" json:"case_id"`
+	Title       string           `db:"title" json:"title"`
+	DocType     *string          `db:"doc_type" json:"doc_type,omitempty"`
+	FilePath    *string          `db:"file_path" json:"file_path,omitempty"`
+	FileSize    *int             `db:"file_size" json:"file_size,omitempty"`
+	MimeType    *string          `db:"mime_type" json:"mime_type,omitempty"`
+	AIExtracted *json.RawMessage `db:"ai_extracted" json:"ai_extracted,omitempty"`
+	UploadedBy  *uuid.UUID       `db:"uploaded_by" json:"uploaded_by,omitempty"`
+	CreatedAt   time.Time        `db:"created_at" json:"created_at"`
+	UpdatedAt   time.Time        `db:"updated_at" json:"updated_at"`
+}

backend/internal/models/party.go

@@ -0,0 +1,17 @@
+package models
+
+import (
+	"encoding/json"
+
+	"github.com/google/uuid"
+)
+
+type Party struct {
+	ID             uuid.UUID       `db:"id" json:"id"`
+	TenantID       uuid.UUID       `db:"tenant_id" json:"tenant_id"`
+	CaseID         uuid.UUID       `db:"case_id" json:"case_id"`
+	Name           string          `db:"name" json:"name"`
+	Role           *string         `db:"role" json:"role,omitempty"`
+	Representative *string         `db:"representative" json:"representative,omitempty"`
+	ContactInfo    json.RawMessage `db:"contact_info" json:"contact_info"`
+}

backend/internal/models/tenant.go

@@ -0,0 +1,30 @@
+package models
+
+import (
+	"encoding/json"
+	"time"
+
+	"github.com/google/uuid"
+)
+
+type Tenant struct {
+	ID        uuid.UUID       `db:"id" json:"id"`
+	Name      string          `db:"name" json:"name"`
+	Slug      string          `db:"slug" json:"slug"`
+	Settings  json.RawMessage `db:"settings" json:"settings"`
+	CreatedAt time.Time       `db:"created_at" json:"created_at"`
+	UpdatedAt time.Time       `db:"updated_at" json:"updated_at"`
+}
+
+type UserTenant struct {
+	UserID    uuid.UUID `db:"user_id" json:"user_id"`
+	TenantID  uuid.UUID `db:"tenant_id" json:"tenant_id"`
+	Role      string    `db:"role" json:"role"`
+	CreatedAt time.Time `db:"created_at" json:"created_at"`
+}
+
+// TenantWithRole is a Tenant joined with the user's role in that tenant.
+type TenantWithRole struct {
+	Tenant
+	Role string `db:"role" json:"role"`
+}

backend/internal/router/router.go

@@ -0,0 +1,75 @@
+package router
+
+import (
+	"encoding/json"
+	"net/http"
+
+	"github.com/jmoiron/sqlx"
+
+	"mgit.msbls.de/m/KanzlAI-mGMT/internal/auth"
+	"mgit.msbls.de/m/KanzlAI-mGMT/internal/handlers"
+	"mgit.msbls.de/m/KanzlAI-mGMT/internal/services"
+)
+
+func New(db *sqlx.DB, authMW *auth.Middleware) http.Handler {
+	mux := http.NewServeMux()
+
+	// Services
+	tenantSvc := services.NewTenantService(db)
+
+	// Middleware
+	tenantResolver := auth.NewTenantResolver(tenantSvc)
+
+	// Handlers
+	tenantH := handlers.NewTenantHandler(tenantSvc)
+
+	// Public routes
+	mux.HandleFunc("GET /health", handleHealth(db))
+
+	// Authenticated API routes
+	api := http.NewServeMux()
+
+	// Tenant management (no tenant resolver — these operate across tenants)
+	api.HandleFunc("POST /api/tenants", tenantH.CreateTenant)
+	api.HandleFunc("GET /api/tenants", tenantH.ListTenants)
+	api.HandleFunc("GET /api/tenants/{id}", tenantH.GetTenant)
+	api.HandleFunc("POST /api/tenants/{id}/invite", tenantH.InviteUser)
+	api.HandleFunc("DELETE /api/tenants/{id}/members/{uid}", tenantH.RemoveMember)
+	api.HandleFunc("GET /api/tenants/{id}/members", tenantH.ListMembers)
+
+	// Tenant-scoped routes (require tenant context)
+	scoped := http.NewServeMux()
+	scoped.HandleFunc("GET /api/cases", placeholder("cases"))
+	scoped.HandleFunc("GET /api/deadlines", placeholder("deadlines"))
+	scoped.HandleFunc("GET /api/appointments", placeholder("appointments"))
+	scoped.HandleFunc("GET /api/documents", placeholder("documents"))
+
+	// Wire: auth -> tenant routes go directly, scoped routes get tenant resolver
+	api.Handle("/api/", tenantResolver.Resolve(scoped))
+
+	mux.Handle("/api/", authMW.RequireAuth(api))
+
+	return mux
+}
+
+func handleHealth(db *sqlx.DB) http.HandlerFunc {
+	return func(w http.ResponseWriter, r *http.Request) {
+		if err := db.Ping(); err != nil {
+			w.WriteHeader(http.StatusServiceUnavailable)
+			json.NewEncoder(w).Encode(map[string]string{"status": "error", "error": err.Error()})
+			return
+		}
+		w.Header().Set("Content-Type", "application/json")
+		json.NewEncoder(w).Encode(map[string]string{"status": "ok"})
+	}
+}
+
+func placeholder(resource string) http.HandlerFunc {
+	return func(w http.ResponseWriter, r *http.Request) {
+		w.Header().Set("Content-Type", "application/json")
+		json.NewEncoder(w).Encode(map[string]string{
+			"status":   "not_implemented",
+			"resource": resource,
+		})
+	}
+}

backend/internal/services/tenant_service.go

@@ -0,0 +1,211 @@
+package services
+
+import (
+	"context"
+	"database/sql"
+	"fmt"
+
+	"github.com/google/uuid"
+	"github.com/jmoiron/sqlx"
+
+	"mgit.msbls.de/m/KanzlAI-mGMT/internal/models"
+)
+
+type TenantService struct {
+	db *sqlx.DB
+}
+
+func NewTenantService(db *sqlx.DB) *TenantService {
+	return &TenantService{db: db}
+}
+
+// Create creates a new tenant and assigns the creator as owner.
+func (s *TenantService) Create(ctx context.Context, userID uuid.UUID, name, slug string) (*models.Tenant, error) {
+	tx, err := s.db.BeginTxx(ctx, nil)
+	if err != nil {
+		return nil, fmt.Errorf("begin transaction: %w", err)
+	}
+	defer tx.Rollback()
+
+	var tenant models.Tenant
+	err = tx.QueryRowxContext(ctx,
+		`INSERT INTO tenants (name, slug) VALUES ($1, $2) RETURNING id, name, slug, settings, created_at, updated_at`,
+		name, slug,
+	).StructScan(&tenant)
+	if err != nil {
+		return nil, fmt.Errorf("insert tenant: %w", err)
+	}
+
+	_, err = tx.ExecContext(ctx,
+		`INSERT INTO user_tenants (user_id, tenant_id, role) VALUES ($1, $2, 'owner')`,
+		userID, tenant.ID,
+	)
+	if err != nil {
+		return nil, fmt.Errorf("assign owner: %w", err)
+	}
+
+	if err := tx.Commit(); err != nil {
+		return nil, fmt.Errorf("commit: %w", err)
+	}
+
+	return &tenant, nil
+}
+
+// ListForUser returns all tenants the user belongs to.
+func (s *TenantService) ListForUser(ctx context.Context, userID uuid.UUID) ([]models.TenantWithRole, error) {
+	var tenants []models.TenantWithRole
+	err := s.db.SelectContext(ctx, &tenants,
+		`SELECT t.id, t.name, t.slug, t.settings, t.created_at, t.updated_at, ut.role
+		 FROM tenants t
+		 JOIN user_tenants ut ON ut.tenant_id = t.id
+		 WHERE ut.user_id = $1
+		 ORDER BY t.name`,
+		userID,
+	)
+	if err != nil {
+		return nil, fmt.Errorf("list tenants: %w", err)
+	}
+	return tenants, nil
+}
+
+// GetByID returns a single tenant. The caller must verify the user has access.
+func (s *TenantService) GetByID(ctx context.Context, tenantID uuid.UUID) (*models.Tenant, error) {
+	var tenant models.Tenant
+	err := s.db.GetContext(ctx, &tenant,
+		`SELECT id, name, slug, settings, created_at, updated_at FROM tenants WHERE id = $1`,
+		tenantID,
+	)
+	if err == sql.ErrNoRows {
+		return nil, nil
+	}
+	if err != nil {
+		return nil, fmt.Errorf("get tenant: %w", err)
+	}
+	return &tenant, nil
+}
+
+// GetUserRole returns the user's role in a tenant, or empty string if not a member.
+func (s *TenantService) GetUserRole(ctx context.Context, userID, tenantID uuid.UUID) (string, error) {
+	var role string
+	err := s.db.GetContext(ctx, &role,
+		`SELECT role FROM user_tenants WHERE user_id = $1 AND tenant_id = $2`,
+		userID, tenantID,
+	)
+	if err == sql.ErrNoRows {
+		return "", nil
+	}
+	if err != nil {
+		return "", fmt.Errorf("get user role: %w", err)
+	}
+	return role, nil
+}
+
+// FirstTenantForUser returns the user's first tenant (by name), used as default.
+func (s *TenantService) FirstTenantForUser(ctx context.Context, userID uuid.UUID) (*uuid.UUID, error) {
+	var tenantID uuid.UUID
+	err := s.db.GetContext(ctx, &tenantID,
+		`SELECT t.id FROM tenants t
+		 JOIN user_tenants ut ON ut.tenant_id = t.id
+		 WHERE ut.user_id = $1
+		 ORDER BY t.name LIMIT 1`,
+		userID,
+	)
+	if err == sql.ErrNoRows {
+		return nil, nil
+	}
+	if err != nil {
+		return nil, fmt.Errorf("first tenant: %w", err)
+	}
+	return &tenantID, nil
+}
+
+// ListMembers returns all members of a tenant.
+func (s *TenantService) ListMembers(ctx context.Context, tenantID uuid.UUID) ([]models.UserTenant, error) {
+	var members []models.UserTenant
+	err := s.db.SelectContext(ctx, &members,
+		`SELECT user_id, tenant_id, role, created_at FROM user_tenants WHERE tenant_id = $1 ORDER BY created_at`,
+		tenantID,
+	)
+	if err != nil {
+		return nil, fmt.Errorf("list members: %w", err)
+	}
+	return members, nil
+}
+
+// InviteByEmail looks up a user by email in auth.users and adds them to the tenant.
+func (s *TenantService) InviteByEmail(ctx context.Context, tenantID uuid.UUID, email, role string) (*models.UserTenant, error) {
+	// Look up user in Supabase auth.users
+	var userID uuid.UUID
+	err := s.db.GetContext(ctx, &userID,
+		`SELECT id FROM auth.users WHERE email = $1`,
+		email,
+	)
+	if err == sql.ErrNoRows {
+		return nil, fmt.Errorf("no user found with email %s", email)
+	}
+	if err != nil {
+		return nil, fmt.Errorf("lookup user: %w", err)
+	}
+
+	// Check if already a member
+	var exists bool
+	err = s.db.GetContext(ctx, &exists,
+		`SELECT EXISTS(SELECT 1 FROM user_tenants WHERE user_id = $1 AND tenant_id = $2)`,
+		userID, tenantID,
+	)
+	if err != nil {
+		return nil, fmt.Errorf("check membership: %w", err)
+	}
+	if exists {
+		return nil, fmt.Errorf("user is already a member of this tenant")
+	}
+
+	var ut models.UserTenant
+	err = s.db.QueryRowxContext(ctx,
+		`INSERT INTO user_tenants (user_id, tenant_id, role) VALUES ($1, $2, $3)
+		 RETURNING user_id, tenant_id, role, created_at`,
+		userID, tenantID, role,
+	).StructScan(&ut)
+	if err != nil {
+		return nil, fmt.Errorf("invite user: %w", err)
+	}
+
+	return &ut, nil
+}
+
+// RemoveMember removes a user from a tenant. Cannot remove the last owner.
+func (s *TenantService) RemoveMember(ctx context.Context, tenantID, userID uuid.UUID) error {
+	// Check if the user being removed is an owner
+	role, err := s.GetUserRole(ctx, userID, tenantID)
+	if err != nil {
+		return fmt.Errorf("check role: %w", err)
+	}
+	if role == "" {
+		return fmt.Errorf("user is not a member of this tenant")
+	}
+
+	if role == "owner" {
+		// Count owners — prevent removing the last one
+		var ownerCount int
+		err := s.db.GetContext(ctx, &ownerCount,
+			`SELECT COUNT(*) FROM user_tenants WHERE tenant_id = $1 AND role = 'owner'`,
+			tenantID,
+		)
+		if err != nil {
+			return fmt.Errorf("count owners: %w", err)
+		}
+		if ownerCount <= 1 {
+			return fmt.Errorf("cannot remove the last owner of a tenant")
+		}
+	}
+
+	_, err = s.db.ExecContext(ctx,
+		`DELETE FROM user_tenants WHERE user_id = $1 AND tenant_id = $2`,
+		userID, tenantID,
+	)
+	if err != nil {
+		return fmt.Errorf("remove member: %w", err)
+	}
+
+	return nil
+}