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/imgcache.py

@@ -9,11 +9,11 @@ The cache directory (``cache.CACHE_DIR``) is shared with the subtitle cache (see
 """
 
 from fastapi import Response
-from sqlalchemy import select
 from sqlalchemy.orm import Session
 
 from app.models import PlexCollection, PlexItem, PlexSeason, PlexShow
 from app.plex.cache import CACHE_DIR
+from app.plex.lookups import by_rating_key
 
 _EXT_CT = [("jpg", "image/jpeg"), ("png", "image/png"), ("webp", "image/webp")]
 _CT_EXT = {ct: ext for ext, ct in _EXT_CT}
@@ -25,24 +25,16 @@ SIZES = {"thumb": (400, 600), "art": (1280, 720)}
 def key_for(db: Session, rating_key: str, variant: str) -> str | None:
     """The stored Plex image path for a known rating_key (item/show/season). Only mirrored keys
     are proxyable — this is not an open image proxy."""
-    it = db.execute(
-        select(PlexItem).where(PlexItem.rating_key == rating_key)
-    ).scalars().first()
+    it = by_rating_key(db, PlexItem, rating_key)
     if it is not None:
         return it.art_key if variant == "art" else it.thumb_key
-    sh = db.execute(
-        select(PlexShow).where(PlexShow.rating_key == rating_key)
-    ).scalars().first()
+    sh = by_rating_key(db, PlexShow, rating_key)
     if sh is not None:
         return sh.art_key if variant == "art" else sh.thumb_key
-    se = db.execute(
-        select(PlexSeason).where(PlexSeason.rating_key == rating_key)
-    ).scalars().first()
+    se = by_rating_key(db, PlexSeason, rating_key)
     if se is not None:
         return se.thumb_key
-    col = db.execute(
-        select(PlexCollection).where(PlexCollection.rating_key == rating_key)
-    ).scalars().first()
+    col = by_rating_key(db, PlexCollection, rating_key)
     if col is not None:
         return col.thumb_key
     return None