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.

demo/.env.example

@@ -0,0 +1,5 @@
+# Demo admin credentials (use strong password in production!)
+DEMO_ADMIN_PASS=admin12345
+
+# JWT secret (change this!)
+JWT_SECRET=demo-secret-change-me-in-production