feat(messages): 고속접수(fast) 엔드포인트 전환 및 GroupCount.sendTotal 매핑 수정 - #34

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