Name uploaded and downloaded files J-<id>.<ext>

Files added through the upload pipeline and Download to Library are
renamed to their J-ID right after indexing, so every new library file is
traceable by its identifier (scanned files keep their existing names).
index_file now returns the created location so callers can rename it;
the location record is updated to the new path.

backend/apps/library/views.py

@@ -212,7 +212,7 @@ class DownloadToLibraryView(APIView):
         destination = services.unique_destination(folder, name)
         try:
             services.download_file(url, destination)
-            item, _, _ = services.index_file(destination, folder)
+            item, _, location, _ = services.index_file(destination, folder)
         except Exception as exc:  # noqa: BLE001 - report download failures
             destination.unlink(missing_ok=True)
             return Response(
@@ -239,6 +239,7 @@ class DownloadToLibraryView(APIView):
 
         if update_fields:
             item.save(update_fields=update_fields + ["updated_at"])
+        services.rename_location_to_j_id(item, location)
         return Response(
             MediaItemSerializer(item, context={"request": request}).data,
             status=status.HTTP_201_CREATED,