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/base/base_handler.py

@@ -376,6 +376,9 @@ class BaseMessageHandler:
 
         使用 asyncio 后台任务执行数据库更新，避免阻塞流式传输
 
+        注意：TTFB（首字节时间）由 StreamContext.record_first_byte_time() 记录，
+        并在最终 record_success 时传递到数据库，避免重复记录导致数据不一致。
+
         Args:
             request_id: 请求 ID，如果不传则使用 self.request_id
         """
@@ -407,6 +410,9 @@ class BaseMessageHandler:
 
         使用 asyncio 后台任务执行数据库更新，避免阻塞流式传输
 
+        注意：TTFB（首字节时间）由 StreamContext.record_first_byte_time() 记录，
+        并在最终 record_success 时传递到数据库，避免重复记录导致数据不一致。
+
         Args:
             ctx: 流式上下文，包含 provider_name 和 mapped_model
         """