refactor(server/coverart): provider interface takes ArtistRef/AlbumRef

Changes the AlbumCoverProvider and ArtistArtProvider interfaces from
MBID-only string parameters to ref-struct signatures so name-based
providers (Deezer, Last.fm in upcoming tasks) can act on rows
without MBIDs.

Today, 53 of 213 artists in the dev DB have NULL MBIDs — featured
artists, remixers, and compilation contributors created from
track-tag splits where the primary-artist MBID is in the file but
collaborator IDs aren't. These rows are unreachable by the current
MBID-only chain. Refactoring the interface unblocks future
name-based providers from acting on them.

TheAudioDB and MBCAA keep MBID-only behavior internally: they return
ErrNotFound when ref.MBID is empty rather than attempting a
name-based fallback. The MBID guard inside EnrichAlbum/EnrichArtist
is removed; providers receive the ref unconditionally and decide
whether they can act on it.

GetAlbumWithFirstTrackPath now JOINs artists to return artist_name
alongside the existing fields, supporting AlbumRef construction.

No behavioral change today (only TheAudioDB + MBCAA registered, both
keep MBID-only behavior). The change unblocks T3 (Deezer) and T4
(Last.fm) which can now register and act on every artist/album
regardless of MBID presence.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

internal/db/queries/covers.sql

@@ -23,11 +23,15 @@ SELECT a.id, a.mbid, a.title, a.artist_id
  LIMIT $1;
 
 -- name: GetAlbumWithFirstTrackPath :one
--- Returns the album row and one of its track file paths (used to derive
--- the album directory). Pick lowest position track for determinism.
+-- Returns the album row, the artist's name (used by name-based art
+-- providers like Deezer/Last.fm), and one of its track file paths
+-- (used to derive the album directory for sidecar writes). Pick
+-- lowest position track for determinism.
 SELECT a.id, a.mbid, a.title, a.cover_art_path, a.cover_art_source,
+       ar.name AS artist_name,
        t.file_path AS track_file_path
   FROM albums a
+  JOIN artists ar ON ar.id = a.artist_id
   LEFT JOIN tracks t ON t.album_id = a.id
  WHERE a.id = $1
  ORDER BY t.disc_number NULLS LAST, t.track_number NULLS LAST, t.id