feat(playlists): gocron-based per-user scheduler

Builds the per-user daily-at-03:00-local scheduler for #392 Half B.
Uses github.com/go-co-op/gocron/v2 with WithLocation(userTZ) for
each user's job. Hourly reconciliation pass keeps the in-memory
job set in sync with the active-users query.

Start sequence: clear stale in_flight rows; register a daily job
for every active user at their stored timezone; fire a one-shot
runOnce to catch up missed schedules during downtime; start gocron.

Stop drains and shuts down the gocron loop.

Refresh(ctx, userID) removes the user's existing job (if any) and
registers a fresh one at their current timezone — wired into the
PUT /api/me/timezone handler and new-user registration in the next
commit.

Server struct gains PlaylistScheduler; main.go constructs it after
the eventbus and threads it through to api.Mount. The existing
StartSystemPlaylistCron call stays for one more commit so we don't
have a window where no scheduler is running; Task 3 deletes it.

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

cmd/minstrel/main.go

@@ -159,6 +159,19 @@ func run() error {
 	lidarrReconciler := lidarrrequests.NewReconciler(pool, lidarrCfg, logger.With("component", "lidarr"), bus)
 	go lidarrReconciler.Run(ctx)
 
+	// Per-user system-playlist scheduler (#392 Half B). Fires each
+	// active user's daily build at 03:00 in their stored timezone.
+	// Replaces the 24h-anchored cron loop (removed in the next commit
+	// of this arc).
+	playlistScheduler, err := playlists.NewScheduler(pool, logger.With("component", "playlist_scheduler"), cfg.Storage.DataDir)
+	if err != nil {
+		return fmt.Errorf("init playlist scheduler: %w", err)
+	}
+	if err := playlistScheduler.Start(ctx); err != nil {
+		return fmt.Errorf("start playlist scheduler: %w", err)
+	}
+	defer playlistScheduler.Stop()
+
 	// Ensure DataDir exists before any service tries to write into it.
 	// Fatal on failure: a non-writable data_dir silently breaks every
 	// downstream cache (playlist covers, artist art, album-cover fallback)
@@ -184,6 +197,7 @@ func run() error {
 		AllowPlaintextPassword: cfg.Subsonic.AllowPlaintextPassword,
 	}, 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,