Add demo mode with hourly reset, example trips & demo banner

DEMO_MODE=true enables: auto-seeded admin + demo user, 3 example trips
(Tokyo, Barcelona, Wien), hourly reset of demo user data, one-click
demo login, visible banner with feature info. Zero behavior change
when DEMO_MODE is not set.

server/src/index.js

@@ -115,7 +115,9 @@ const PORT = process.env.PORT || 3001;
 const server = app.listen(PORT, () => {
   console.log(`NOMAD API running on port ${PORT}`);
   console.log(`Environment: ${process.env.NODE_ENV || 'development'}`);
+  if (process.env.DEMO_MODE === 'true') console.log('Demo mode: ENABLED');
   scheduler.start();
+  scheduler.startDemoReset();
   const { setupWebSocket } = require('./websocket');
   setupWebSocket(server);
 });