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/management/commands/scan_files.py

@@ -67,7 +67,9 @@ class Command(BaseCommand):
                     continue
                 path = Path(root) / name
                 try:
-                    item, created_item, created_location = index_file(path, folder)
+                    item, created_item, _location, created_location = index_file(
+                        path, folder
+                    )
                 except OSError as exc:
                     skipped += 1
                     self.stderr.write(f"Skipped {path}: {exc}")