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/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"])
+	}
+}