fix(service): tolerate concurrent pending-to-uploading status race in UploadChunk

When multiple chunk uploads race on the pending→uploading transition, ignore ErrRecordNotFound from UpdateSessionStatus since another request already completed the update.

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-openagent)

Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

internal/service/upload_service.go

@@ -190,9 +190,11 @@ func (s *UploadService) UploadChunk(ctx context.Context, sessionID int64, chunkI
 
 	if session.Status == "pending" {
 		if err := s.uploadStore.UpdateSessionStatus(ctx, sessionID, "uploading"); err != nil {
+			if !errors.Is(err, gorm.ErrRecordNotFound) {
 				return fmt.Errorf("update status to uploading: %w", err)
 			}
 		}
+	}
 
 	return nil
 }