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/routes/plex/playback.py

@@ -16,6 +16,7 @@ from fastapi import (
 )
 from fastapi.responses import FileResponse
 from pydantic import BaseModel
+from sqlalchemy import select
 from sqlalchemy.orm import Session
 
 from app.auth import current_user, resolved_user_id
@@ -207,7 +208,11 @@ def _push_watch(
         return
     # Capture the row's freshest timestamp NOW so the background push can tell if a newer local edit
     # lands before it settles synced_to_plex (which would otherwise strand that newer value).
-    st = db.query(PlexState).filter_by(user_id=user_id, item_id=item_id).first()
+    st = db.execute(
+        select(PlexState).where(
+            PlexState.user_id == user_id, PlexState.item_id == item_id
+        )
+    ).scalars().first()
     background.add_task(
         plex_watch.push_state_to_plex,
         user_id,
@@ -246,7 +251,11 @@ def item_progress(
     duration = int(body.duration_seconds)
     final = body.final
     now = datetime.now(timezone.utc)
-    st = db.query(PlexState).filter_by(user_id=user.id, item_id=it.id).first()
+    st = db.execute(
+        select(PlexState).where(
+            PlexState.user_id == user.id, PlexState.item_id == it.id
+        )
+    ).scalars().first()
     was_watched = st is not None and st.status == "watched"
 
     near_end = duration > 0 and position > duration - _FINISH_MARGIN_S
@@ -316,7 +325,11 @@ def item_state(
     status = body.status
     if status not in ("new", "watched", "hidden"):
         raise HTTPException(status_code=400, detail="Invalid status")
-    st = db.query(PlexState).filter_by(user_id=user.id, item_id=it.id).first()
+    st = db.execute(
+        select(PlexState).where(
+            PlexState.user_id == user.id, PlexState.item_id == it.id
+        )
+    ).scalars().first()
     prev_status = st.status if st is not None else "new"
     if status == "new":
         if st is not None:
@@ -351,10 +364,12 @@ def _bulk_state(
     now = datetime.now(timezone.utc)
     existing = {
         s.item_id: s
-        for s in db.query(PlexState).filter(
-            PlexState.user_id == user_id,
-            PlexState.item_id.in_([e.id for e in eps] or [0]),
-        )
+        for s in db.execute(
+            select(PlexState).where(
+                PlexState.user_id == user_id,
+                PlexState.item_id.in_([e.id for e in eps] or [0]),
+            )
+        ).scalars()
     }
     pushable = plex_watch.link_for_push(db, user_id) is not None
     to_push: list[tuple[int, str, str]] = []
@@ -397,11 +412,15 @@ 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.query(PlexShow).filter_by(rating_key=str(rating_key)).first()
+    sh = db.execute(
+        select(PlexShow).where(PlexShow.rating_key == str(rating_key))
+    ).scalars().first()
     if sh is None:
         raise HTTPException(status_code=404, detail="Unknown Plex show")
     watched = body.watched
-    eps = db.query(PlexItem).filter_by(show_id=sh.id, kind="episode").all()
+    eps = db.execute(
+        select(PlexItem).where(PlexItem.show_id == sh.id, PlexItem.kind == "episode")
+    ).scalars().all()
     changed = _bulk_state(background, db, user.id, eps, watched)
     return {"changed": changed, "watched": watched}
 
@@ -415,11 +434,15 @@ 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.query(PlexSeason).filter_by(rating_key=str(rating_key)).first()
+    se = db.execute(
+        select(PlexSeason).where(PlexSeason.rating_key == str(rating_key))
+    ).scalars().first()
     if se is None:
         raise HTTPException(status_code=404, detail="Unknown Plex season")
     watched = body.watched
-    eps = db.query(PlexItem).filter_by(season_id=se.id, kind="episode").all()
+    eps = db.execute(
+        select(PlexItem).where(PlexItem.season_id == se.id, PlexItem.kind == "episode")
+    ).scalars().all()
     changed = _bulk_state(background, db, user.id, eps, watched)
     return {"changed": changed, "watched": watched}