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/reader/models.py

@@ -11,12 +11,14 @@ from sqlalchemy import (
     DateTime,
     Float,
     ForeignKey,
+    Index,
     Integer,
     JSON,
     String,
     Text,
     UniqueConstraint,
     func,
+    text,
 )
 from sqlalchemy.orm import Mapped, mapped_column, relationship
 
@@ -93,6 +95,15 @@ class Playlist(Base, TimestampMixin, UpdatedAtMixin):
     sync phases; the foundation phase only uses local playlists.)"""
 
     __tablename__ = "playlists"
+    __table_args__ = (
+        # One Watch later per user, held by the database: two first saves at once each made one.
+        Index(
+            "uq_playlist_watch_later",
+            "user_id",
+            unique=True,
+            postgresql_where=text("kind = 'watch_later'"),
+        ),
+    )
 
     id: Mapped[int] = mapped_column(primary_key=True)
     user_id: Mapped[int] = mapped_column(ForeignKey("users.id", ondelete="CASCADE"), index=True)