test: seedGenre gives each artist one artist row and one album

seedTrack mints a new artist per call when there is no mbid, so the genre
fixtures held one artist per track and the candidate query's artist_ids
counted 30 where the test meant 6.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/playlists/genre_mixes_db_test.go

@@ -15,13 +15,18 @@ import (
 )
 
 // seedGenre creates artists × perArtist tracks tagged genre, each artist on
-// its own album, and returns the tracks.
+// its own album, and returns the tracks. seedTrack mints a new artist on
+// every call (an artist without an mbid has no conflict key), so each
+// artist's first track creates it and the rest join that track's album.
 func seedGenre(t *testing.T, pool *pgxpool.Pool, genre string, artists, perArtist int) []dbq.Track {
 	t.Helper()
 	var out []dbq.Track
 	for a := 0; a < artists; a++ {
-		for k := 0; k < perArtist; k++ {
-			tr := seedTrack(t, pool, fmt.Sprintf("%s %d-%d", genre, a, k), fmt.Sprintf("%s Artist %d", genre, a))
+		first := seedTrack(t, pool, fmt.Sprintf("%s %d-0", genre, a), fmt.Sprintf("%s Artist %d", genre, a))
+		setGenre(t, pool, first.ID, genre)
+		out = append(out, first)
+		for k := 1; k < perArtist; k++ {
+			tr := seedTrackForArtist(t, pool, fmt.Sprintf("%s %d-%d", genre, a, k), first.AlbumID, first.ArtistID)
 			setGenre(t, pool, tr.ID, genre)
 			out = append(out, tr)
 		}