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/albums.sql

@@ -85,3 +85,14 @@ SELECT COUNT(*) FROM albums;
 -- Used by request-progress reporting to count how many albums have been
 -- ingested for a matched artist while a request is still in flight.
 SELECT COUNT(*) FROM albums WHERE artist_id = $1;
+
+-- name: DeleteAlbumIfEmpty :one
+-- 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".
+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;