feat(playlists): wire Refresh into PUT /api/me/timezone + registration

Completes the server side of #392 Half B.

PUT /api/me/timezone now calls scheduler.Refresh(ctx, userID) after
the DB write so the rescheduled daily-at-03:00-local job takes
effect synchronously. Failure to refresh is logged but doesn't
undo the DB write — the hourly reconciliation pass would pick it
up within an hour regardless.

POST /api/auth/register calls Refresh after successful user
insert so brand-new users get scheduled immediately rather than
waiting for the hourly pass to discover them.

system_cron.go deleted: the new scheduler subsumes its
responsibilities. The StartSystemPlaylistCron call in main.go is
also removed. Server restart now runs the new scheduler's startup
recovery + catch-up instead.

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

cmd/minstrel/main.go

@@ -198,7 +198,6 @@ func run() error {
 	}, cfg.Events, cfg.Recommendation, cfg.Storage.DataDir, cfg.Branding, coverEnricher, cfg.Library.CoverArtBackfillCap, coverSettings, scanner, scanCfg, scheduler)
 	srv.Bus = bus
 	srv.PlaylistScheduler = playlistScheduler
-	playlists.StartSystemPlaylistCron(ctx, pool, logger.With("component", "system_playlist_cron"), cfg.Storage.DataDir)
 	httpServer := &http.Server{
 		Addr:              cfg.Server.Address,
 		Handler:           srv.Router(),