feat: update demo properties

Author: tkahng

src/main/resources/application-demo.properties

@@ -28,3 +28,6 @@ app.jwt.secret=${JWT_SECRET:bG9jYWxEZXZTZWNyZXRLZXlUaGF0SXNBdExlYXN0MzJCeXRlc0xv
 # Google OAuth2
 spring.security.oauth2.client.registration.google.client-id=${GOOGLE_CLIENT_ID:local-client-id}
 spring.security.oauth2.client.registration.google.client-secret=${GOOGLE_CLIENT_SECRET:local-client-secret}
+
+# Admin notifications (optional)
+app.admin-notification-email=${ADMIN_NOTIFICATION_EMAIL:}