feat(library): backlog status, rating, notes and priority per game

New games start as BACKLOG, or PLAYING when they have playtime. The sync
never changes a status later. PUT /api/library/games/{appid}/backlog sets
all four fields; /api/library/games filters by status and name (q).

src/main/resources/db/migration/V3__backlog_fields.sql

@@ -0,0 +1,12 @@
+-- Per-user backlog tracking. The sync sets status only when it inserts a game.
+ALTER TABLE user_game
+    ADD COLUMN status   VARCHAR(16),
+    ADD COLUMN rating   INTEGER  CHECK (rating BETWEEN 1 AND 5),
+    ADD COLUMN notes    VARCHAR(2000),
+    ADD COLUMN priority VARCHAR(8);
+
+UPDATE user_game SET status = CASE WHEN playtime_minutes > 0 THEN 'PLAYING' ELSE 'BACKLOG' END;
+
+ALTER TABLE user_game ALTER COLUMN status SET NOT NULL;
+
+CREATE INDEX idx_user_game_user_status ON user_game (user_id, status) WHERE removed_at IS NULL;