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/coverart/enricher.go

@@ -89,18 +89,24 @@ func (e *Enricher) EnrichAlbum(ctx context.Context, albumID pgtype.UUID) error {
 		}
 	}
 
-	// MBID guard — providers can't fetch without one.
-	if row.Mbid == nil || *row.Mbid == "" {
-		return nil // leave NULL; mbid backfill or future scan can heal
+	// Build the ref. Every provider receives it and decides whether it
+	// can act on the fields available (MBID-only providers return
+	// ErrNotFound when MBID is empty; name-based providers fall back to
+	// ArtistName + AlbumTitle).
+	ref := AlbumRef{
+		ArtistName: row.ArtistName,
+		AlbumTitle: row.Title,
+	}
+	if row.Mbid != nil {
+		ref.MBID = *row.Mbid
 	}
-	mbid := *row.Mbid
 
 	// Provider chain. allWere404 tracks whether every provider
 	// returned ErrNotFound — only then do we settle to 'none'. Any
 	// transient failure leaves the row NULL for next-pass retry.
 	allWere404 := true
 	for _, provider := range e.settings.EnabledAlbumProviders() {
-		body, perr := provider.FetchAlbumCover(ctx, mbid)
+		body, perr := provider.FetchAlbumCover(ctx, ref)
 		if perr == nil {
 			if row.TrackFilePath == nil || *row.TrackFilePath == "" {
 				// Defensive: shouldn't happen if MBID was set on an