userfront 런타임 BACKEND_URL fallback 수정

userfront/lib/core/services/audit_service.dart

@@ -1,18 +1,10 @@
 import 'dart:convert';
 import 'package:flutter/foundation.dart';
 import 'package:http/http.dart' as http;
-import 'package:flutter_dotenv/flutter_dotenv.dart';
+import 'runtime_env.dart';
 
 class AuditService {
-  static String _envOrDefault(String key, String fallback) {
-    if (!dotenv.isInitialized) {
-      return fallback;
-    }
-    return dotenv.env[key] ?? fallback;
-  }
-
-  static String get _baseUrl =>
-      _envOrDefault('BACKEND_URL', 'https://sso.hmac.kr');
+  static String get _baseUrl => runtimeBackendUrl();
 
   static Future<void> logEvent({
     required String userId,