Rename unused loop var to silence ruff B007

`sub_name` is unused — the recursion only needs the path. Rename to `_sub_name`
to satisfy ruff's B007 check.

core/auto_import_worker.py

@@ -490,7 +490,7 @@ class AutoImportWorker:
 
             # Otherwise recurse into non-disc subdirs (disc folders only
             # ever attach to a parent album, never stand alone).
-            for sub_name, sub_path in non_disc_subdirs:
+            for _sub_name, sub_path in non_disc_subdirs:
                 self._scan_directory(sub_path, candidates, staging_root=staging_root)
 
     def _is_folder_stable(self, candidate: FolderCandidate) -> bool: