fix(lint): rename unused now param in produceDiscover to _

revive unused-parameter: produceDiscover keys off dateStr, not now,
but must keep the uniform systemPlaylistProducer signature. Blank
the unused param (param names don't affect func-type identity).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

internal/playlists/system.go

@@ -370,7 +370,7 @@ func produceSeedMixes(
 // (logged, yields no Discover) to match the prior behavior.
 func produceDiscover(
 	ctx context.Context, q *dbq.Queries, logger *slog.Logger,
-	userID pgtype.UUID, dateStr string, now time.Time,
+	userID pgtype.UUID, dateStr string, _ time.Time,
 ) ([]builtPlaylist, error) {
 	tracks, derr := buildDiscoverCandidates(ctx, q, logger, userID, dateStr)
 	if derr != nil {