refactor: 알람 세션 단일 진실 원천 기반 + 저장소 계층 (Phase A·B1·B2) + 알람 지연 버그 수정 - #367

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/