fix: stage zip downloads to temp file and improve file serving headers

Write zip to a temporary file before serving to enable correct content-length, range requests, and proper cache-control headers. Additionally, handle negative object sizes by falling back to file metadata for content-length.

backend/libs/services/upload.go

@@ -1028,9 +1028,13 @@ func (s *UploadService) RecordDownload(boxID string) error {
 	})
 }
 
-func (s *UploadService) WriteZip(w io.Writer, box Box) error {
+func (s *UploadService) WriteZip(w io.Writer, box Box) (err error) {
 	archive := zip.NewWriter(w)
-	defer archive.Close()
+	defer func() {
+		if closeErr := archive.Close(); err == nil {
+			err = closeErr
+		}
+	}()
 
 	for _, file := range box.Files {
 		object, err := s.OpenFileObject(context.Background(), box, file)