fix: address critical security and data integrity issues from cross-audit

- Add expire_time > NOW() filter to findActive() preventing stale subscriptions
- Add express-rate-limit on login endpoints (user: 10/15min, admin: 5/15min)
- Add production guard for default admin credentials
- Fix BLE bindDevice userId encoding (uint32 instead of hexToBytes on numeric)
- Wrap adminCreate in transaction to prevent race condition
- Add settings cache invalidation after admin saves
- Read trial_days from settings instead of hardcoding 7
- Fix double JSON.stringify in commandDao.finish call
- Cancel stale pending bindings before creating new ones
- Reduce token refresh grace period from 3 days to 1 day
- Fix subscribe-success to fetch expiry from server (correct for renewals)
- Add keep-alive name property to DashboardView and SettingsView
- Fix BLE disconnect() to preserve listener registrations across reconnects

server/package.json

@@ -13,6 +13,7 @@
     "cos-nodejs-sdk-v5": "^2.14.7",
     "dotenv": "^16.4.5",
     "express": "^5.2.1",
+    "express-rate-limit": "^8.5.0",
     "jsonwebtoken": "^9.0.2",
     "mysql2": "^3.11.3"
   }