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/gemini/relay-gemini.go

@@ -1297,12 +1297,11 @@ func geminiStreamHandler(c *gin.Context, info *relaycommon.RelayInfo, resp *http
 	var imageCount int
 	responseText := strings.Builder{}
 
-	helper.StreamScannerHandler(c, resp, info, func(data string) bool {
+	helper.StreamScannerHandler(c, resp, info, func(data string, sr *helper.StreamResult) {
 		var geminiResponse dto.GeminiChatResponse
-		err := common.UnmarshalJsonStr(data, &geminiResponse)
-		if err != nil {
-			logger.LogError(c, "error unmarshalling stream response: "+err.Error())
-			return false
+		if err := common.UnmarshalJsonStr(data, &geminiResponse); err != nil {
+			sr.Stop(fmt.Errorf("unmarshal: %w", err))
+			return
 		}
 
 		if len(geminiResponse.Candidates) == 0 && geminiResponse.PromptFeedback != nil && geminiResponse.PromptFeedback.BlockReason != nil {
@@ -1327,7 +1326,9 @@ func geminiStreamHandler(c *gin.Context, info *relaycommon.RelayInfo, resp *http
 			*usage = mappedUsage
 		}
 
-		return callback(data, &geminiResponse)
+		if !callback(data, &geminiResponse) {
+			sr.Stop(fmt.Errorf("gemini callback stopped"))
+		}
 	})
 
 	if imageCount != 0 {