fix: a first watch state and a first Watch later survive two requests at once

The first checkpoint of an item and the final flush beside it both read no
state and both inserted; the loser failed on uq_plex_user_item. Two first saves
each made a Watch later list. Both are created by an ON CONFLICT DO NOTHING
insert and then read; a partial unique index holds one Watch later per user, and
the migration folds an existing second list into the first. Plex keeps one
per-item state map (rollup.item_state_map).

backend/app/plex/rollup.py

@@ -8,6 +8,7 @@ from datetime import datetime, timezone
 from sqlalchemy import and_, case, func, or_, select
 from sqlalchemy.orm import Session, aliased
 
+from app.models import User
 from app.userscope import owned_by
 from app.plex.models import PlexItem, PlexState
 from app.plex.cards import ShowState
@@ -106,7 +107,7 @@ def state_map(db: Session, user_id: int, show_ids: list[int]) -> dict[int, ShowS
     return {r.show_id: show_state(r.total, r.watched, r.inprog) for r in rows}
 
 
-def item_state_map(db: Session, user_id: int, item_ids: list[int]) -> dict[int, PlexState]:
+def item_state_map(db: Session, user_id: User | int, item_ids: list[int]) -> dict[int, PlexState]:
     """A user's PlexState rows for a set of leaf items (movies or episodes), keyed by item_id — the
     leaf analogue of `state_map` (which rolls a show's episodes up to a `ShowState`). One batched
     fetch, so the caller serializes cards without a per-card query. Empty in → empty dict (no `IN ()`)."""