feat(demo): reset the demo instance every 20 minutes with a countdown

DEMO_RESET_MINUTES (default 20, 0 = off) resets the database to the
sample data. Demo accounts get fixed IDs so sessions survive; requests
hold a read lock so the reset never runs under a half-finished request
(streams excluded). DEMO_RESET goes to every connection, /auth/status
announces the next reset. The app shows "✨ Demo · Reset in MM:SS" in
place of the version, then clears cached answers, remounts the views,
reloads in the background and shows a short notice; if no event
arrives (long polling) it asks the server after the countdown.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/api/server.go

@@ -28,6 +28,8 @@ type Server struct {
 
 	leases   *collab.LeaseManager // block leases and open sessions of notes
 	notifier *collab.Notifier     // wakes long-poll requests on note changes
+
+	demo demoState // periodic reset in demo mode (demo.go)
 }
 
 func NewServer(cfg *config.Config, store *storage.Store, wsHub *ws.Hub, staticFS fs.FS) *Server {
@@ -46,6 +48,9 @@ func NewServer(cfg *config.Config, store *storage.Store, wsHub *ws.Hub, staticFS
 	}
 	go srv.startAlarmTicker()
 	go srv.startCompaction()
+	if cfg.DemoMode && cfg.DemoResetMinutes > 0 {
+		go srv.startDemoResetTicker()
+	}
 	return srv
 }
 
@@ -70,6 +75,7 @@ func (s *Server) Router() http.Handler {
 	r := chi.NewRouter()
 
 	r.Use(middleware.RealIP)
+	r.Use(s.demoResetGate)
 	r.Use(middleware.Recoverer)
 	r.Use(s.apiLoggingMiddleware)
 	r.Use(cors.Handler(cors.Options{