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/stream_status.go

@@ -0,0 +1,112 @@
+package common
+
+import (
+	"fmt"
+	"strings"
+	"sync"
+	"time"
+)
+
+type StreamEndReason string
+
+const (
+	StreamEndReasonNone        StreamEndReason = ""
+	StreamEndReasonDone        StreamEndReason = "done"
+	StreamEndReasonTimeout     StreamEndReason = "timeout"
+	StreamEndReasonClientGone  StreamEndReason = "client_gone"
+	StreamEndReasonScannerErr  StreamEndReason = "scanner_error"
+	StreamEndReasonHandlerStop StreamEndReason = "handler_stop"
+	StreamEndReasonEOF         StreamEndReason = "eof"
+	StreamEndReasonPanic       StreamEndReason = "panic"
+	StreamEndReasonPingFail    StreamEndReason = "ping_fail"
+)
+
+const maxStreamErrorEntries = 20
+
+type StreamErrorEntry struct {
+	Message   string
+	Timestamp time.Time
+}
+
+type StreamStatus struct {
+	EndReason  StreamEndReason
+	EndError   error
+	endOnce    sync.Once
+
+	mu         sync.Mutex
+	Errors     []StreamErrorEntry
+	ErrorCount int
+}
+
+func NewStreamStatus() *StreamStatus {
+	return &StreamStatus{}
+}
+
+func (s *StreamStatus) SetEndReason(reason StreamEndReason, err error) {
+	if s == nil {
+		return
+	}
+	s.endOnce.Do(func() {
+		s.EndReason = reason
+		s.EndError = err
+	})
+}
+
+func (s *StreamStatus) RecordError(msg string) {
+	if s == nil {
+		return
+	}
+	s.mu.Lock()
+	defer s.mu.Unlock()
+	s.ErrorCount++
+	if len(s.Errors) < maxStreamErrorEntries {
+		s.Errors = append(s.Errors, StreamErrorEntry{
+			Message:   msg,
+			Timestamp: time.Now(),
+		})
+	}
+}
+
+func (s *StreamStatus) HasErrors() bool {
+	if s == nil {
+		return false
+	}
+	s.mu.Lock()
+	defer s.mu.Unlock()
+	return s.ErrorCount > 0
+}
+
+func (s *StreamStatus) TotalErrorCount() int {
+	if s == nil {
+		return 0
+	}
+	s.mu.Lock()
+	defer s.mu.Unlock()
+	return s.ErrorCount
+}
+
+func (s *StreamStatus) IsNormalEnd() bool {
+	if s == nil {
+		return true
+	}
+	return s.EndReason == StreamEndReasonDone ||
+		s.EndReason == StreamEndReasonEOF ||
+		s.EndReason == StreamEndReasonHandlerStop
+}
+
+func (s *StreamStatus) Summary() string {
+	if s == nil {
+		return "StreamStatus<nil>"
+	}
+	b := &strings.Builder{}
+	fmt.Fprintf(b, "reason=%s", s.EndReason)
+	if s.EndError != nil {
+		fmt.Fprintf(b, " end_error=%q", s.EndError.Error())
+	}
+	s.mu.Lock()
+	if s.ErrorCount > 0 {
+		fmt.Fprintf(b, " soft_errors=%d", s.ErrorCount)
+	}
+	s.mu.Unlock()
+	return b.String()
+}