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/ws/hub.go

@@ -371,6 +371,26 @@ func (h *Hub) BroadcastToUser(userID, event string, payload interface{}) {
 	}
 }
 
+// BroadcastSystem sends an event to every connection. Only for events without user data (e.g. the
+// demo reset notice).
+func (h *Hub) BroadcastSystem(event string) {
+	data, err := json.Marshal(domain.WSMessage{Event: event, Timestamp: domain.NowISO()})
+	if err != nil {
+		return
+	}
+	h.mu.RLock()
+	defer h.mu.RUnlock()
+	if h.closed {
+		return
+	}
+	for client := range h.allClients {
+		client.trySend(data)
+	}
+	for sseClient := range h.sseClients {
+		sseClient.trySend(data)
+	}
+}
+
 // BroadcastToUsers sends one event to every connection of the given users (duplicates are ignored).
 func (h *Hub) BroadcastToUsers(userIDs []string, event string, payload interface{}) {
 	seen := make(map[string]bool, len(userIDs))