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_migration_0112.py

@@ -0,0 +1,68 @@
+"""Migration 0112's data step, DB-lane: a user who already has two Watch later lists keeps the oldest,
+with the other's videos moved in after its own (one it already held is not added twice), and the
+one-per-user index then refuses a second list.
+
+The schema half is checked by test_migrations.py; a video lost on the way is invisible there.
+"""
+
+from datetime import datetime, timezone
+
+import pytest
+from alembic import command
+from alembic.config import Config
+from sqlalchemy import text
+from sqlalchemy.exc import IntegrityError
+
+from app.catalog.models import Channel, Video
+from app.db import engine
+
+
+def test_a_second_watch_later_is_folded_into_the_first(db, make_user):
+    user, other = make_user(), make_user()
+    db.add(Channel(id="UCm", title="M"))
+    now = datetime.now(timezone.utc)
+    for v in ("v1", "v2", "v3"):
+        db.add(Video(id=v, channel_id="UCm", published_at=now, duration_seconds=60))
+    db.commit()
+    cfg = Config("alembic.ini")
+    command.downgrade(cfg, "0111_audit_request_id")
+    try:
+        with engine.begin() as conn:
+            ids = conn.execute(
+                text(
+                    "INSERT INTO playlists (user_id, name, kind, position) VALUES"
+                    " (:u, 'Watch later', 'watch_later', 0), (:u, 'Watch later', 'watch_later', 1),"
+                    " (:o, 'Watch later', 'watch_later', 0) RETURNING id"
+                ),
+                {"u": user.id, "o": other.id},
+            ).scalars()
+            first, second, theirs = list(ids)
+            conn.execute(
+                text(
+                    "INSERT INTO playlist_items (playlist_id, video_id, position) VALUES"
+                    " (:a, 'v1', 0), (:b, 'v2', 0), (:b, 'v1', 1), (:c, 'v3', 0)"
+                ),
+                {"a": first, "b": second, "c": theirs},
+            )
+    finally:
+        command.upgrade(cfg, "head")
+
+    with engine.connect() as conn:
+        lists = conn.execute(
+            text("SELECT id, user_id FROM playlists WHERE kind = 'watch_later' ORDER BY id")
+        ).all()
+        assert [tuple(r) for r in lists] == [(first, user.id), (theirs, other.id)]
+        items = conn.execute(
+            text(
+                "SELECT playlist_id, video_id, position FROM playlist_items ORDER BY playlist_id, position"
+            )
+        ).all()
+        assert [tuple(r) for r in items] == [(first, "v1", 0), (first, "v2", 1), (theirs, "v3", 0)]
+    with pytest.raises(IntegrityError), engine.begin() as conn:
+        conn.execute(
+            text(
+                "INSERT INTO playlists (user_id, name, kind, position)"
+                " VALUES (:u, 'Watch later', 'watch_later', 2)"
+            ),
+            {"u": user.id},
+        )