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.

relay/common/relay_info.go

@@ -162,6 +162,8 @@ type RelayInfo struct {
 	// 若为空，调用 GetFinalRequestRelayFormat 会回退到 RequestConversionChain 的最后一项或 RelayFormat。
 	FinalRequestRelayFormat types.RelayFormat
 
+	StreamStatus *StreamStatus
+
 	ThinkingContentInfo
 	TokenCountMeta
 	*ClaudeConvertInfo