Two bugs on the Downloaded browse surface (UR-055/UR-056):
1. Grouping — browsing a downloaded *library* listed individual leaves
(songs, episodes) instead of their containers. The library-level match in
`get_downloaded_items` selected every downloaded item on the server; add a
NOT EXISTS clause so the top level shows only albums/series/movies, with
leaves still reachable by drilling in. Regression tests for music + TV.
2. "Loading your downloads…" hung on large libraries. The disk-usage
partiality query did an OR-based self-join over the entire synced catalog
(O(items^2), unindexable). Narrow it to downloaded containers first via a
CTE, and add the missing idx_items_season index (migration 020 + base
schema) — parent_id/album_id/series_id were already indexed.
Also annotate the existing backend tests that cover the IT-016/IT-017
end-to-end offline-listing scenarios with their trace IDs.