fix(plex): address S1 code-review findings

- Restore synchronize_session=False on the remove-bulk Core delete (the 2.0
  default is 'auto'; keep the pre-conversion contract explicit).
- _episode_nav: list(...scalars()) instead of [r[0] for r in ...all()], matching
  the house style used across the conversion.
- Extract app/plex/lookups.by_rating_key() and migrate the get-by-rating_key
  pattern at 13 sites (imgcache x4, _shared.item_or_404, playback show/season,
  browse x6).
- Coverage: add TestFacetsWithData (the converted wsq aggregation with real
  data — the highest-risk untested builder), TestPlaylistAndCollectionEndpoints
  (playlist CRUD + collections list), and an item-state playback test.

Full suite 256 passed, 2 skipped; ruff clean.

backend/app/plex/lookups.py

@@ -0,0 +1,11 @@
+"""Small shared Plex row lookups, so the get-by-rating_key pattern lives in one place."""
+from sqlalchemy import select
+from sqlalchemy.orm import Session
+
+
+def by_rating_key(db: Session, model, rating_key: str):
+    """The mirrored row for a Plex rating_key, or None. `model` is any of the rating_key-keyed Plex
+    tables (PlexItem / PlexShow / PlexSeason / PlexCollection)."""
+    return db.execute(
+        select(model).where(model.rating_key == str(rating_key))
+    ).scalars().first()