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/tests/test_plex_service.py

@@ -12,7 +12,7 @@ from datetime import datetime, timezone
 
 from sqlalchemy import select
 
-from app.plex import library, queries, service
+from app.plex import library, queries, rollup, service
 from app.plex.models import PlexCollection, PlexItem, PlexLibrary, PlexShow, PlexState
 from app.plex.query import LibraryFilters
 
@@ -71,9 +71,9 @@ def test_a_users_watch_state_is_theirs_alone(db, make_user):
     _state(db, owner, it, status="watched")
     assert queries.state(db, owner, it.id).status == "watched"
     assert queries.state(db, stranger, it.id) is None
-    assert queries.states(db, stranger, [it.id]) == {}
-    assert set(queries.states(db, owner, [it.id])) == {it.id}
-    assert queries.states(db, owner, []) == {}
+    assert rollup.item_state_map(db, stranger, [it.id]) == {}
+    assert set(rollup.item_state_map(db, owner, [it.id])) == {it.id}
+    assert rollup.item_state_map(db, owner, []) == {}
 
 
 def test_the_grid_shows_the_callers_state_and_hides_only_what_they_hid(db, make_user):
@@ -421,3 +421,18 @@ def test_only_the_owners_own_link_pushes_their_watch_state(db, make_user):
     db.commit()
     assert watch_sync.link_for_push(db, owner.id).id == link.id
     assert watch_sync.link_for_push(db, other.id) is None
+
+
+def test_a_first_write_that_lost_the_race_updates_the_winners_row(db, make_user):
+    """The first checkpoint of an item and the `final` flush beside it both read "no state"; the one
+    that writes second must land on the row the first made, not fail on `uq_plex_user_item`."""
+    user = make_user()
+    it = _movie(db, _lib(db), "m-race")
+    _state(db, user, it, position_seconds=10)  # the request that won, committed
+    st = service.checkpoint(db, user, it.id, None, 42, NOW, dirty=True)  # read "none" before it
+    assert (st.position_seconds, st.synced_to_plex) == (42, False)
+    st = service.set_status(db, user, it.id, None, "hidden")
+    assert st.status == "hidden"
+    db.commit()
+    rows = db.scalars(select(PlexState).where(PlexState.item_id == it.id)).all()
+    assert [(r.position_seconds, r.status) for r in rows] == [(42, "hidden")]