feat: add database schema and backend foundation

Part 1 - Database (kanzlai schema in Supabase):
- Tenant-scoped tables: tenants, user_tenants, cases, parties,
  deadlines, appointments, documents, case_events
- Global reference tables: proceeding_types, deadline_rules, holidays
- RLS policies on all tenant-scoped tables
- Seed: UPC proceeding types, 32 deadline rules (INF/CCR/REV/PI/APP),
  ZPO civil rules (Berufung, Revision, Einspruch), 2026 holidays

Part 2 - Backend skeleton:
- config: env var loading (DATABASE_URL, SUPABASE_*, ANTHROPIC_API_KEY)
- db: sqlx connection pool with kanzlai search_path
- auth: JWT verification middleware adapted from youpc.org, context helpers
- models: Go structs for all tables with sqlx/json tags
- router: route registration with auth middleware, /health + placeholder API routes
- Updated main.go to wire everything together

backend/internal/router/router.go

@@ -0,0 +1,50 @@
+package router
+
+import (
+	"encoding/json"
+	"net/http"
+
+	"mgit.msbls.de/m/KanzlAI-mGMT/internal/auth"
+
+	"github.com/jmoiron/sqlx"
+)
+
+func New(db *sqlx.DB, authMW *auth.Middleware) http.Handler {
+	mux := http.NewServeMux()
+
+	// Public routes
+	mux.HandleFunc("GET /health", handleHealth(db))
+
+	// Authenticated API routes
+	api := http.NewServeMux()
+	api.HandleFunc("GET /api/cases", placeholder("cases"))
+	api.HandleFunc("GET /api/deadlines", placeholder("deadlines"))
+	api.HandleFunc("GET /api/appointments", placeholder("appointments"))
+	api.HandleFunc("GET /api/documents", placeholder("documents"))
+
+	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,
+		})
+	}
+}