fix(upload): add 30s timeout to background chunk cleanup goroutine

The cleanup goroutine used context.Background() with no timeout, so if
MinIO accepted TCP connections but never responded, the goroutine would
block indefinitely. Now uses context.WithTimeout to prevent leaks.

internal/service/upload_service.go

@@ -357,8 +357,9 @@ func (s *UploadService) CompleteUpload(ctx context.Context, sessionID int64) (*m
 			keys[i] = fmt.Sprintf("%schunk_%05d", minioPrefix, i)
 		}
 		go func() {
-			bgCtx := context.Background()
-			if delErr := s.storage.RemoveObjects(bgCtx, s.cfg.Bucket, keys, storage.RemoveObjectsOptions{}); delErr != nil {
+			ctx, cancel := context.WithTimeout(context.Background(), 30*time.Second)
+			defer cancel()
+			if delErr := s.storage.RemoveObjects(ctx, s.cfg.Bucket, keys, storage.RemoveObjectsOptions{}); delErr != nil {
 				s.logger.Warn("delete temp chunks", zap.Error(delErr))
 			}
 		}()