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>

frontend/src/lib/types.ts

@@ -10,6 +10,10 @@ export interface AuthStatus {
   sharingMode?: SharingMode;
   // "Passwort vergessen?" works (SMTP and BASE_URL configured)
   passwordReset?: boolean;
+  // Demo mode with periodic reset: next reset (ISO) and seconds until then
+  demoNextReset?: string;
+  demoResetInSeconds?: number;
+  demoResetMinutes?: number;
   // Impressum / Datenschutz of this instance (LEGAL_* environment variables)
   legal?: {
     imprintUrl?: string;