fix(playlists): use CRON_TZ-prefixed cron expressions for per-job timezone

gocron v2's WithLocation is a SchedulerOption (process-wide), not a
JobOption — there's no per-job location knob. To get per-user
timezones we use a cron expression with the CRON_TZ= prefix, which
the underlying robfig/cron parser honors:

  CRON_TZ=America/New_York 0 3 * * *

Fires at 03:00 in the named zone every day. Same DST-correctness as
the original WithLocation approach.

Fall-back to UTC moves inline (was validateTimezoneOrUTC); kept the
helper because the test file still exercises it.

Caught by go vet on CI after slice 2 pushed:
"cannot use gocron.WithLocation(loc) (value of type
gocron.SchedulerOption) as gocron.JobOption value"

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

internal/playlists/scheduler.go

@@ -134,6 +134,12 @@ func (s *Scheduler) Refresh(ctx context.Context, userID pgtype.UUID) error {
 }
 
 // registerLocked replaces the user's job entry. Caller must hold s.mu.
+//
+// gocron v2 doesn't expose a per-job location option — WithLocation is
+// a SchedulerOption that affects every job. To get per-user timezones
+// we use a cron expression with the CRON_TZ= prefix, which the
+// underlying robfig/cron parser honors. The expression "CRON_TZ=$tz
+// 0 3 * * *" fires at 03:00 in the named zone every day.
 func (s *Scheduler) registerLocked(ctx context.Context, userID pgtype.UUID, tz string) error {
 	if existing, ok := s.jobs[userID]; ok {
 		if err := s.gocron.RemoveJob(existing); err != nil {
@@ -142,18 +148,21 @@ func (s *Scheduler) registerLocked(ctx context.Context, userID pgtype.UUID, tz s
 		}
 		delete(s.jobs, userID)
 	}
-	loc := validateTimezoneOrUTC(tz)
+	// Fall back to UTC if the stored tz is unparseable so a corrupted
+	// DB row doesn't error out registration.
+	resolvedTZ := "UTC"
+	if _, err := validateTimezone(tz); err == nil {
+		resolvedTZ = tz
+	}
+	cronExpr := fmt.Sprintf("CRON_TZ=%s 0 %d * * *", resolvedTZ, dailyRebuildHour)
 	job, err := s.gocron.NewJob(
-		gocron.DailyJob(1, gocron.NewAtTimes(
-			gocron.NewAtTime(dailyRebuildHour, 0, 0),
-		)),
+		gocron.CronJob(cronExpr, false /* withSeconds */),
 		gocron.NewTask(func() {
 			if err := BuildSystemPlaylists(ctx, s.pool, s.logger, userID, time.Now(), s.dataDir); err != nil {
 				s.logger.Warn("scheduler: build failed",
 					"user_id", uuidStringPL(userID), "err", err)
 			}
 		}),
-		gocron.WithLocation(loc),
 	)
 	if err != nil {
 		return fmt.Errorf("register daily job: %w", err)