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/people.py

@@ -5,7 +5,7 @@ re-sync (plex.sync), so the two paths never diverge."""
 
 from urllib.parse import quote
 
-from sqlalchemy import text
+from sqlalchemy import select, text
 from sqlalchemy.dialects.postgresql import insert as pg_insert
 from sqlalchemy.orm import Session
 
@@ -55,7 +55,10 @@ def enrich_row_people(db: Session, row, cast: list[dict]) -> bool:
         dirty = True
     # Headshot cache: upsert name -> proxied thumb (a later fetch fills a prior NULL; names without a
     # whitelisted photo still get a row so a card can show a placeholder).
-    existing = {p.name: p for p in db.query(PlexPerson).filter(PlexPerson.name.in_(names))}
+    existing = {
+        p.name: p
+        for p in db.execute(select(PlexPerson).where(PlexPerson.name.in_(names))).scalars()
+    }
     missing: dict[str, str | None] = {}
     for c in cast:
         nm, th = c.get("name"), c.get("thumb")