feat(server): emit playlist.system_rebuilt on daily + manual system-playlist rebuild

The daily 03:00 scheduler rebuild (and the manual refresh endpoint) replace
a user's system playlists + You-might-like rows but published no event, so a
client left open across the rebuild served yesterday's snapshot until a
manual reload — the stale-tab case behind #968. Add a user-scoped
playlist.system_rebuilt event (envelope {kind,user_id,data:{}}) from both the
scheduler (bus threaded into NewScheduler) and handleSystemPlaylistRefresh.
Clients consume it to invalidate home / system-playlist views and proactively
re-pull a stale active queue. Issue #968.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

cmd/minstrel/main.go

@@ -242,7 +242,12 @@ func run() error {
 	// 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)
+	playlistScheduler, err := playlists.NewScheduler(
+		pool,
+		logger.With("component", "playlist_scheduler"),
+		cfg.Storage.DataDir,
+		bus,
+	)
 	if err != nil {
 		return fmt.Errorf("init playlist scheduler: %w", err)
 	}