refactor(plex): convert service-layer db.query() to 2.0 select() (wave 1)

Mechanical 1.x->2.0 conversion of the self-contained plex modules, behaviour-
equivalent (the characterization net + existing plex tests stay green, 39
passed): app/plex/{sync(15),watch_sync(9),people(1),imgcache(4)} and
app/routes/plex/{admin(4),playback(8),_shared(1)}. Query builders become
select(); .first()/.all()->execute().scalars(); a bulk .update() ->
execute(update()); a single-column .first() -> db.scalar().

browse.py (61) + rollup.agg_subquery (the atomic unit with unified_library)
remain for wave 2.

backend/app/plex/imgcache.py

@@ -9,6 +9,7 @@ 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
@@ -24,16 +25,24 @@ 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.query(PlexItem).filter_by(rating_key=rating_key).first()
+    it = db.execute(
+        select(PlexItem).where(PlexItem.rating_key == rating_key)
+    ).scalars().first()
     if it is not None:
         return it.art_key if variant == "art" else it.thumb_key
-    sh = db.query(PlexShow).filter_by(rating_key=rating_key).first()
+    sh = db.execute(
+        select(PlexShow).where(PlexShow.rating_key == rating_key)
+    ).scalars().first()
     if sh is not None:
         return sh.art_key if variant == "art" else sh.thumb_key
-    se = db.query(PlexSeason).filter_by(rating_key=rating_key).first()
+    se = db.execute(
+        select(PlexSeason).where(PlexSeason.rating_key == rating_key)
+    ).scalars().first()
     if se is not None:
         return se.thumb_key
-    col = db.query(PlexCollection).filter_by(rating_key=rating_key).first()
+    col = db.execute(
+        select(PlexCollection).where(PlexCollection.rating_key == rating_key)
+    ).scalars().first()
     if col is not None:
         return col.thumb_key
     return None