CoverArtProxy now checks three sources in order:
1. Local library (instant) — matches by album+artist name against
the release_groups/cover_art tables. Albums the user already
owns show their local cover art immediately.
2. Disk cache (instant) — previously fetched CAA thumbnails.
3. Cover Art Archive (network) — fetches and caches to disk.
Library index is built once on first access (sync.Once) from a
single SQL query joining release_groups → cover_art → artists.
Keyed by lowercased 'album\x00artist' for exact name matching.
GetThumbnail now takes (mbid, albumName, artistName) so the proxy
can check the library before falling back to CAA. Frontend passes
the album title and artist credit from the search result.
yonlu
0761cff408