feat: AtchaV2 Phase 8 — 갱신 채널: FCM 사일런트 푸시(가드) + 폴링 폴백 + 하드닝 - #346

Node.js 20 is deprecated. The following actions target Node.js 20 but are being forced to run on Node.js 24: actions/checkout@v4. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/