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/_shared.py

@@ -3,20 +3,18 @@
 import logging
 
 from fastapi import HTTPException
-from sqlalchemy import select
 from sqlalchemy.orm import Session
 
 from app.models import (
     PlexItem,
 )
+from app.plex.lookups import by_rating_key
 
 log = logging.getLogger("siftlode.plex")
 
 
 def item_or_404(db: Session, rating_key: str) -> PlexItem:
-    it = db.execute(
-        select(PlexItem).where(PlexItem.rating_key == str(rating_key))
-    ).scalars().first()
+    it = by_rating_key(db, PlexItem, rating_key)
     if it is None:
         raise HTTPException(status_code=404, detail="Unknown Plex item")
     return it