feat(db): cascade-cleanup queries for M7 #372 track removal

DeleteTrack returns album_id + artist_id so the calling service can
chain the album-empty / artist-empty cascade. DeleteAlbumIfEmpty and
DeleteArtistIfEmpty are no-ops when other rows still reference the
parent — the service treats pgx.ErrNoRows as "not orphaned, skip".

internal/db/dbq/albums.sql.go

@@ -47,6 +47,30 @@ func (q *Queries) CountAlbumsMatching(ctx context.Context, dollar_1 string) (int
 	return count, err
 }
 
+const deleteAlbumIfEmpty = `-- name: DeleteAlbumIfEmpty :one
+DELETE FROM albums a
+WHERE a.id = $1
+  AND NOT EXISTS (SELECT 1 FROM tracks t WHERE t.album_id = a.id)
+RETURNING a.id, a.artist_id
+`
+
+type DeleteAlbumIfEmptyRow struct {
+	ID       pgtype.UUID
+	ArtistID pgtype.UUID
+}
+
+// M7 #372: deletes the album row only when it has no remaining tracks.
+// Used after a track delete to tidy up orphaned albums. RETURNING gives
+// us the artist_id so the service can chain into the artist-empty check.
+// Returns no rows when the album still has tracks; the service treats
+// pgx.ErrNoRows as "album not orphaned, skip cascade".
+func (q *Queries) DeleteAlbumIfEmpty(ctx context.Context, id pgtype.UUID) (DeleteAlbumIfEmptyRow, error) {
+	row := q.db.QueryRow(ctx, deleteAlbumIfEmpty, id)
+	var i DeleteAlbumIfEmptyRow
+	err := row.Scan(&i.ID, &i.ArtistID)
+	return i, err
+}
+
 const getAlbumByArtistAndTitle = `-- name: GetAlbumByArtistAndTitle :one
 SELECT id, title, sort_title, artist_id, release_date, mbid, cover_art_path, created_at, updated_at FROM albums WHERE artist_id = $1 AND title = $2 LIMIT 1
 `