- by_rating_key: generic-typed (type[_M] -> _M | None) so call sites keep their
Model | None static type.
- Migrate the last 2 open-coded PlexCollection-by-rating_key lookups (sync.py
resync_collection + delete_collection_local) to by_rating_key; zero open-coded
rating_key lookups now remain in the package.
- Add test_playlist_bulk_add_and_remove covering the bulk add/remove endpoints
and pinning the remove-bulk synchronize_session=False path.
Full suite 258 passed, 2 skipped; ruff clean.