fix: every first watch state survives a write beside it, and a folded Watch later keeps its pin

A season marked watched, a pull from Plex and a YouTube resume point made their first row by a read
then an add, and failed on the unique key when a first checkpoint or the watched mark made it in
between. They go through the same insert-then-read as the checkpoint (the resume point is one
upsert); a pull that meets a local change still waiting for its push leaves it.

Folding duplicate Watch later lists moved their videos but not a Home pin, which the cascade then
deleted; the first pin of a dropped copy moves onto the kept list.

Marking one notice read trimmed before its own flag was flushed, so the trim left one past the cap.
The route test now crosses the cap and reads the result from a session of its own.

backend/app/plex/service.py

@@ -302,16 +302,29 @@ def mark_finished(db: Session, user_id: int, item_id: int, now: datetime) -> boo
     return False
 
 
-def _state_row(db: Session, user: User, item_id: int, st: PlexState | None) -> PlexState:
+def state_row(
+    db: Session,
+    user: User | int,
+    item_id: int,
+    st: PlexState | None = None,
+    *,
+    synced: bool = False,
+) -> PlexState:
     """The user's state row on this item, made if the read before found none. Made by the insert
     itself (ON CONFLICT DO NOTHING) and then read: the first checkpoint of an item and the `final`
     flush beside it routinely both read "none", and a plain add let the second one fail on
-    `uq_plex_user_item` — a 500, and the push it was carrying lost."""
+    `uq_plex_user_item` — a 500, and the push it was carrying lost. Every first row goes through
+    here: a checkpoint, a status, a season marked at once, a pull from Plex. `synced` is what a row
+    made now says about Plex; a row another writer made first keeps its own."""
     if st is not None:
         return st
     db.execute(
         pg_insert(PlexState)
-        .values(user_id=user.id, item_id=item_id)
+        .values(
+            user_id=user if isinstance(user, int) else user.id,
+            item_id=item_id,
+            synced_to_plex=synced,
+        )
         .on_conflict_do_nothing(constraint="uq_plex_user_item")
     )
     made = queries.state(db, user, item_id)
@@ -342,7 +355,7 @@ def checkpoint(
 ) -> PlexState:
     """Record a resume position. `dirty` marks it not yet mirrored to Plex — set before a push, so
     the reconcile re-pushes it if the push fails and the push can settle it once Plex confirms."""
-    st = _state_row(db, user, item_id, st)
+    st = state_row(db, user, item_id, st)
     st.position_seconds = position
     st.progress_updated_at = now
     if dirty:
@@ -360,7 +373,7 @@ def set_status(
     db: Session, user: User, item_id: int, st: PlexState | None, status: str
 ) -> PlexState:
     """Set a watched or hidden status; watched also clears the resume point."""
-    st = _state_row(db, user, item_id, st)
+    st = state_row(db, user, item_id, st)
     st.status = status
     if status == "watched":
         st.watched_at = datetime.now(timezone.utc)
@@ -383,9 +396,8 @@ def bulk_state(
         st = existing.get(e.id)
         prev = st.status if st is not None else "new"
         if watched:
-            if st is None:
-                st = PlexState(user_id=user_id, item_id=e.id)
-                db.add(st)
+            st = state_row(db, user_id, e.id, st)
+            prev = st.status  # a first checkpoint beside this may have made it meanwhile
             st.status = "watched"
             st.watched_at = now
             st.position_seconds = 0