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/uploads.py

@@ -55,7 +55,8 @@ def complete_temp_upload(temp, download_url=None):
         except Exception:
             destination.unlink(missing_ok=True)
             raise
-        item, _, _ = services.index_file(destination, folder)
+        item, _, location, _ = services.index_file(destination, folder)
+        services.rename_location_to_j_id(item, location)
         if temp.file:
             temp.file.delete(save=False)
     else:
@@ -66,7 +67,8 @@ def complete_temp_upload(temp, download_url=None):
         )
         with temp.file.open("rb") as source, open(destination, "wb") as target:
             shutil.copyfileobj(source, target)
-        item, _, _ = services.index_file(destination, folder)
+        item, _, location, _ = services.index_file(destination, folder)
+        services.rename_location_to_j_id(item, location)
         temp.file.delete(save=False)
 
     temp.library_item = item