feat: first-time setup flow — no .env required for deployment

- Add GET /auth/status endpoint (returns needsSetup when no admin exists)
- Add POST /auth/setup endpoint (public first-admin registration)
- Add IsAdminExists + SetupInitialAdmin methods to AuthService
- Frontend: detect needsSetup on load, show setup page with admin registration
- Frontend: fall back to login page when setup is already complete
- Docker compose: env_file already optional (required: false), no changes needed
- Bootstrap: auto-detect BOOTSTRAP_CLUSTERS without separate enable flag

backend/cmd/api/main.go

@@ -254,6 +254,8 @@ func setupRouter(
 	// ===== 认证路由 =====
 	api.HandleFunc("/auth/login", authHandler.Login).Methods(http.MethodPost)
 	api.HandleFunc("/auth/refresh", authHandler.RefreshToken).Methods(http.MethodPost)
+	api.HandleFunc("/auth/status", authHandler.AuthStatus).Methods(http.MethodGet)
+	api.HandleFunc("/auth/setup", authHandler.Setup).Methods(http.MethodPost)
 
 	protected := api.PathPrefix("").Subrouter()
 	protected.Use(authMiddleware(authService))