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/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
+}