feat: record stream interruption reasons via StreamStatus

- Add StreamStatus type (relay/common) to track stream end reason
  (done/timeout/client_gone/scanner_error/eof/panic/ping_fail) and
  accumulate soft errors during streaming via sync.Once + sync.Mutex.
- Add StreamResult (relay/helper) as the callback interface: adapters
  call sr.Error() for soft errors, sr.Stop() for fatal, sr.Done() for
  normal completion. No early-return problem — multiple errors per chunk
  are naturally supported.
- Refactor StreamScannerHandler callback from func(string) bool to
  func(string, *StreamResult). All 9 channel adapters updated.
- Write stream_status into log other JSON field (admin-only) with
  status ok/error, end_reason, error_count, and error messages.
- Frontend: display stream status in log detail expansion for admins.

web/src/i18n/locales/en.json

@@ -2678,6 +2678,11 @@
     "请求结束后多退少补": "Adjust after request completion",
     "请求超时，请刷新页面后重新发起 GitHub 登录": "Request timed out, please refresh and restart GitHub login",
     "请求路径": "Request path",
+    "流状态": "Stream Status",
+    "流错误详情": "Stream Error Details",
+    "软错误": "soft errors",
+    "正常": "Normal",
+    "异常": "Abnormal",
     "请求转换": "Request conversion",
     "请求预扣费额度": "Pre-deduction quota for requests",
     "请点击我": "Please click me",