feat: add case + party CRUD with case events (Phase 1B)

- CaseService: list (paginated, filterable), get detail (with parties,
  events, deadline count), create, update, soft-delete (archive)
- PartyService: list by case, create, update, delete
- Auto-create case_events on case creation, status change, party add,
  and case archive
- Auth middleware now resolves tenant_id from user_tenants table
- All operations scoped to tenant_id from auth context

backend/cmd/server/main.go

@@ -22,7 +22,7 @@ func main() {
 	}
 	defer database.Close()
 
-	authMW := auth.NewMiddleware(cfg.SupabaseJWTSecret)
+	authMW := auth.NewMiddleware(cfg.SupabaseJWTSecret, database)
 	handler := router.New(database, authMW)
 
 	log.Printf("Starting KanzlAI API server on :%s", cfg.Port)