feat(server/m7-380): one-shot MBID backfill worker for existing libraries

Adds a boot-time goroutine that walks albums with NULL mbid, re-reads
tags from one track per album via dhowden/tag, and persists album + artist
MBIDs. Healed albums also get their cover_art_source='none' cleared so the
enricher's next batch retries the MBCAA fetch. Caps at 5000 albums per
boot to avoid stalling startup on huge libraries.

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

internal/db/dbq/covers.sql.go

@@ -25,6 +25,20 @@ func (q *Queries) ClearAlbumCover(ctx context.Context, id pgtype.UUID) error {
 	return err
 }
 
+const clearAlbumCoverNone = `-- name: ClearAlbumCoverNone :exec
+UPDATE albums
+   SET cover_art_source = NULL
+ WHERE id = $1 AND cover_art_source = 'none'
+`
+
+// M7 #380: when MBID backfill heals an album, clear its 'none' cover_art_source
+// back to NULL so the enricher's next batch retries the MBCAA fetch.
+// Idempotent — only updates rows currently stamped 'none'.
+func (q *Queries) ClearAlbumCoverNone(ctx context.Context, id pgtype.UUID) error {
+	_, err := q.db.Exec(ctx, clearAlbumCoverNone, id)
+	return err
+}
+
 const countAlbumCoverSources = `-- name: CountAlbumCoverSources :one
 SELECT
     COUNT(*) FILTER (WHERE cover_art_source IS NULL)     ::bigint AS untried,