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/services/auth/service.py

@@ -27,7 +27,7 @@ if not config.jwt_secret_key:
     if config.environment == "production":
         raise ValueError("JWT_SECRET_KEY must be set in production environment!")
     config.jwt_secret_key = secrets.token_urlsafe(32)
-    logger.warning(f"JWT_SECRET_KEY未在环境变量中找到，已生成随机密钥用于开发: {config.jwt_secret_key[:10]}...")
+    logger.warning("JWT_SECRET_KEY未在环境变量中找到，已生成随机密钥用于开发")
     logger.warning("生产环境请设置JWT_SECRET_KEY环境变量!")
 
 JWT_SECRET_KEY = config.jwt_secret_key