refactor: 第三方平台架构改造（Adapter Protocol + Gateway）

Phase 1: 异常体系统一
- 新增 PlatformError / PlatformErrorType 标准定义
- 改造所有 Provider 异常抛出为 PlatformError
- 注册全局 PlatformError exception handler

Phase 2: Adapter Protocol
- 新增 app/ai/adapters/base.py（PlatformAdapter + SyncCapable + TaskCapable + CallbackCapable）
- 新增 app/ai/adapters/constants.py（Method 常量）
- 新增 PlatformConfigLoader（config/platform-config.yaml）

Phase 3: HTTP Client 统一
- ViduProvider 从 aiohttp 迁移到 httpx（注入方式）
- VolcengineCaptionService 改为注入 http_client
- lifespan 统一管理所有 Client 创建和关闭

Phase 4: Gateway 骨架 + Adapter 实现
- 新增 ViduAdapter / VolcengineArkAdapter / VolcengineCaptionAdapter
- 新增 PlatformGateway（call_sync / submit_task / query_task / handle_webhook）
- 新增 LLMGateway（带 Fallback 降级链）
- lifespan 注册所有 Adapter 和 Gateway

Phase 6: 清理与验证
- 从 Settings 移除 VIDU_BASE_URL / VOLCENGINE_BASE_URL
- Provider 改为从 PlatformConfigLoader 读取 base_url
- 清理 volcengine_caption_service 全局单例
- config_loader 默认路径改为 platform-config.yaml
- Scheduler 注入共享 HTTP client
- vidu.py 回调路由使用 Adapter 验签和解析
- ruff 全量通过，应用启动测试通过

python-api/app/scheduler/handlers/subtitle_handler.py

@@ -28,6 +28,14 @@ class SubtitleHandler(AsyncHandler):
     slot_key = SLOT_KEY
     max_slots = MAX_SLOTS
 
+    def __init__(self, service: VolcengineCaptionService | None = None):
+        self.service = service
+
+    def _get_service(self) -> VolcengineCaptionService:
+        if self.service is None:
+            self.service = VolcengineCaptionService()
+        return self.service
+
     async def tick(
         self, jobs: list[Any], registry: JobRegistry, slots: SlotManager
     ) -> list[StateChange]:
@@ -45,7 +53,7 @@ class SubtitleHandler(AsyncHandler):
             if volc_task_id:
                 # 轮询
                 try:
-                    service = VolcengineCaptionService()
+                    service = self._get_service()
                     if mode == "auto_align":
                         result = await service.query_auto_align_task(volc_task_id, blocking=False)
                     else: