fix(app-assets): 修复公开API路径双重前缀 + Kong网关路由缺失

问题：移动端配置的开屏图/引导图无效
根因：
1. PublicAppAssetController 和 PublicSystemConfigController 的
   @Controller('api/v1/xxx') 与 NestJS 全局前缀 api/v1 叠加，
   导致实际端点为 api/v1/api/v1/xxx（双重前缀）
2. Kong 网关缺少 /api/v1/app-assets 和 /api/v1/system-config 路由
3. Flutter 端使用 /admin-service/api/v1/xxx 路径，不匹配任何 Kong 路由

修复：
- 后端：Controller 路径去掉 api/v1 前缀，由全局前缀统一添加
- Kong：新增 admin-app-assets-public 和 admin-system-config-public 路由
- Flutter：API 路径改为 /app-assets 和 /system-config/display/settings

受影响文件：
- backend/api-gateway/kong.yml (新增2条路由)
- backend/.../app-asset.controller.ts (Controller路径修正)
- backend/.../system-config.controller.ts (Controller路径修正)
- frontend/.../app_asset_service.dart (API路径修正)
- frontend/.../system_config_service.dart (API路径修正)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

backend/api-gateway/kong.yml

@@ -228,6 +228,14 @@ services:
         paths:
           - /api/v1/mobile/system
         strip_path: false
+      - name: admin-app-assets-public
+        paths:
+          - /api/v1/app-assets
+        strip_path: false
+      - name: admin-system-config-public
+        paths:
+          - /api/v1/system-config
+        strip_path: false
 
   # ---------------------------------------------------------------------------
   # Presence Service - 在线状态服务

backend/services/admin-service/src/api/controllers/app-asset.controller.ts

@@ -229,7 +229,7 @@ export class AdminAppAssetController {
 // =============================================================================
 
 @ApiTags('App Assets (Public)')
-@Controller('api/v1/app-assets')
+@Controller('app-assets')
 export class PublicAppAssetController {
   constructor(private readonly prisma: PrismaService) {}
 

backend/services/admin-service/src/api/controllers/system-config.controller.ts

@@ -173,7 +173,7 @@ export class AdminSystemConfigController {
  * 移动端/公开系统配置控制器
  * 用于 mobile-app 获取展示相关的配置
  */
-@Controller('api/v1/system-config')
+@Controller('system-config')
 export class PublicSystemConfigController {
   constructor(
     @Inject(SYSTEM_CONFIG_REPOSITORY)

frontend/mobile-app/lib/core/services/app_asset_service.dart

@@ -179,7 +179,7 @@ class AppAssetService {
   Future<AppAssetConfig?> _fetchFromApi() async {
     try {
       final response = await _apiClient.get(
-        '/admin-service/api/v1/app-assets',
+        '/app-assets',
       );
       if (response.data is List) {
         return AppAssetConfig.fromList(response.data as List);

frontend/mobile-app/lib/core/services/system_config_service.dart

@@ -77,7 +77,7 @@ class SystemConfigService {
 
     try {
       final response = await _apiClient.get(
-        '/admin-service/api/v1/system-config/display/settings',
+        '/system-config/display/settings',
       );
 
       _cachedDisplaySettings = DisplaySettings.fromJson(response.data);