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/artists.sql.go

@@ -33,6 +33,24 @@ func (q *Queries) CountArtistsMatching(ctx context.Context, dollar_1 string) (in
 	return count, err
 }
 
+const deleteArtistIfEmpty = `-- name: DeleteArtistIfEmpty :one
+DELETE FROM artists a
+WHERE a.id = $1
+  AND NOT EXISTS (SELECT 1 FROM albums al WHERE al.artist_id = a.id)
+  AND NOT EXISTS (SELECT 1 FROM tracks t WHERE t.artist_id = a.id)
+RETURNING a.id
+`
+
+// M7 #372: deletes the artist row only when it has no remaining albums
+// AND no remaining tracks (defensive — tracks can in principle exist
+// without an album, though the scanner doesn't write that shape today).
+func (q *Queries) DeleteArtistIfEmpty(ctx context.Context, id pgtype.UUID) (pgtype.UUID, error) {
+	row := q.db.QueryRow(ctx, deleteArtistIfEmpty, id)
+	var id_2 pgtype.UUID
+	err := row.Scan(&id_2)
+	return id_2, err
+}
+
 const getArtistByID = `-- name: GetArtistByID :one
 SELECT id, name, sort_name, mbid, created_at, updated_at FROM artists WHERE id = $1
 `