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>

internal/api/playlists_system_shuffle.go

@@ -73,6 +73,8 @@ func (h *handlers) handleSystemPlaylistRefresh(w http.ResponseWriter, r *http.Re
 		writeErr(w, apierror.InternalMsg("build failed", err))
 		return
 	}
+	// #968: announce the rebuild so the user's other clients refresh.
+	h.publishSystemRebuilt(user.ID)
 	q := dbq.New(h.pool)
 	v := kind
 	pl, err := q.GetSystemPlaylistByVariantForUser(r.Context(),