fix(plex): address S1 review round-2 findings

- 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.

backend/app/plex/sync.py

@@ -19,6 +19,7 @@ from app import sysconfig
 from app.models import PlexCollection, PlexItem, PlexLibrary, PlexSeason, PlexShow
 from app.plex import people
 from app.plex.client import PlexClient, PlexError, PlexNotConfigured
+from app.plex.lookups import by_rating_key
 
 log = logging.getLogger("siftlode.plex")
 
@@ -382,13 +383,7 @@ def resync_collection(db: Session, plex: PlexClient, lib: PlexLibrary, rk: str)
     collection's membership on the affected member rows."""
     meta = plex.metadata(rk) or {}
     col = _upsert_collection(
-        db,
-        db.execute(
-            select(PlexCollection).where(PlexCollection.rating_key == rk)
-        ).scalars().first(),
-        rk,
-        lib.id,
-        meta,
+        db, by_rating_key(db, PlexCollection, rk), rk, lib.id, meta
     )
     Model = PlexItem if lib.kind == "movie" else PlexShow
     new_members = {str(c.get("ratingKey")) for c in plex.collection_children(rk) if c.get("ratingKey")}
@@ -415,9 +410,7 @@ def delete_collection_local(db: Session, lib: PlexLibrary, rk: str) -> None:
         select(Model).where(Model.collection_keys.contains([rk]))
     ).scalars():
         row.collection_keys = [k for k in (row.collection_keys or []) if k != rk] or None
-    col = db.execute(
-        select(PlexCollection).where(PlexCollection.rating_key == rk)
-    ).scalars().first()
+    col = by_rating_key(db, PlexCollection, rk)
     if col is not None:
         db.delete(col)
     db.flush()