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/routes/plex/playback.py

@@ -30,6 +30,7 @@ from app.models import (
 )
 from app.plex import imgcache, subtitles
 from app.plex import paths as plex_paths
+from app.plex.lookups import by_rating_key
 from app.plex import people as plex_people
 from app.plex import stream as plex_stream
 from app.plex import watch_sync as plex_watch
@@ -412,9 +413,7 @@ def show_state(
     db: Session = Depends(get_db),
 ) -> dict:
     """Mark a whole show watched/unwatched (all its episodes) for the current user + push to Plex."""
-    sh = db.execute(
-        select(PlexShow).where(PlexShow.rating_key == str(rating_key))
-    ).scalars().first()
+    sh = by_rating_key(db, PlexShow, rating_key)
     if sh is None:
         raise HTTPException(status_code=404, detail="Unknown Plex show")
     watched = body.watched
@@ -434,9 +433,7 @@ def season_state(
     db: Session = Depends(get_db),
 ) -> dict:
     """Mark a whole season watched/unwatched (all its episodes) for the current user + push to Plex."""
-    se = db.execute(
-        select(PlexSeason).where(PlexSeason.rating_key == str(rating_key))
-    ).scalars().first()
+    se = by_rating_key(db, PlexSeason, rating_key)
     if se is None:
         raise HTTPException(status_code=404, detail="Unknown Plex season")
     watched = body.watched