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/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)
 	}
 }