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/queries/artists.sql

@@ -64,3 +64,13 @@ LEFT JOIN LATERAL (
 ) cnt ON true
 ORDER BY artists.sort_name, artists.name, artists.id
 LIMIT $1 OFFSET $2;
+
+-- name: DeleteArtistIfEmpty :one
+-- 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).
+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;