feat: add TTFB timeout detection and improve stream handling

- Add stream first byte timeout (TTFB) detection to trigger failover
  when provider responds too slowly (configurable via STREAM_FIRST_BYTE_TIMEOUT)
- Add rate limit fail-open/fail-close strategy configuration
- Improve exception handling in stream prefetch with proper error classification
- Refactor UsageService with shared _prepare_usage_record method
- Add batch deletion for old usage records to avoid long transaction locks
- Update CLI adapters to use proper User-Agent headers for each CLI client
- Add composite indexes migration for usage table query optimization
- Fix streaming status display in frontend to show TTFB during streaming
- Remove sensitive JWT secret logging in auth service

src/api/handlers/gemini_cli/adapter.py

@@ -112,7 +112,7 @@ class GeminiCliAdapter(CliAdapterBase):
     ) -> Tuple[list, Optional[str]]:
         """查询 Gemini API 支持的模型列表（带 CLI User-Agent）"""
         # 复用 GeminiChatAdapter 的实现，添加 CLI User-Agent
-        cli_headers = {"User-Agent": config.internal_user_agent_gemini}
+        cli_headers = {"User-Agent": config.internal_user_agent_gemini_cli}
         if extra_headers:
             cli_headers.update(extra_headers)
         models, error = await GeminiChatAdapter.fetch_models(