feat(app): add vworld geocoding and native ads placeholders

lib/core/constants/api_keys.dart

@@ -29,6 +29,14 @@ class ApiKeys {
   static const String naverLocalSearchEndpoint =
       'https://openapi.naver.com/v1/search/local.json';
 
+  // VWorld 지오코딩 키 (dart-define: VWORLD_API_KEY, base64 권장)
+  static const String _encodedVworldApiKey = String.fromEnvironment(
+    'VWORLD_API_KEY',
+    defaultValue: '',
+  );
+
+  static String get vworldApiKey => _decodeIfNeeded(_encodedVworldApiKey);
+
   static bool areKeysConfigured() {
     return naverClientId.isNotEmpty && naverClientSecret.isNotEmpty;
   }