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/channel/claude/relay-claude.go

@@ -813,12 +813,11 @@ func ClaudeStreamHandler(c *gin.Context, resp *http.Response, info *relaycommon.
 		Usage:        &dto.Usage{},
 	}
 	var err *types.NewAPIError
-	helper.StreamScannerHandler(c, resp, info, func(data string) bool {
+	helper.StreamScannerHandler(c, resp, info, func(data string, sr *helper.StreamResult) {
 		err = HandleStreamResponseData(c, info, claudeInfo, data)
 		if err != nil {
-			return false
+			sr.Stop(err)
 		}
-		return true
 	})
 	if err != nil {
 		return nil, err