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/components/DemoCountdown.tsx

@@ -0,0 +1,38 @@
+import React, { useEffect, useState } from 'react';
+
+interface DemoCountdownProps {
+  deadline: number; // ms timestamp of the next reset, in this browser's clock
+  minutes?: number;
+  className?: string;
+  onElapsed?: () => void;
+}
+
+const fmt = (ms: number) => {
+  const total = Math.max(0, Math.round(ms / 1000));
+  return `${String(Math.floor(total / 60)).padStart(2, '0')}:${String(total % 60).padStart(2, '0')}`;
+};
+
+// "✨ Demo · Reset in MM:SS" in place of the version under the app name while the demo mode resets itself
+export const DemoCountdown: React.FC<DemoCountdownProps> = ({ deadline, minutes, className = '', onElapsed }) => {
+  const [now, setNow] = useState(() => Date.now());
+
+  useEffect(() => {
+    const timer = window.setInterval(() => setNow(Date.now()), 1000);
+    return () => window.clearInterval(timer);
+  }, []);
+
+  const left = deadline - now;
+  useEffect(() => {
+    if (left <= 0) onElapsed?.();
+    // Runs when the deadline passes, not on every tick
+  }, [left <= 0, deadline]);
+
+  return (
+    <span
+      className={`font-mono leading-tight text-accent ${className}`}
+      title={`Diese Test-Instanz wird alle ${minutes || 20} Minuten automatisch auf den Ausgangszustand zurückgesetzt.`}
+    >
+      ✨ Demo · {left > 0 ? `Reset in ${fmt(left)}` : 'wird zurückgesetzt…'}
+    </span>
+  );
+};