feat(admin): 관리자 기능 — 신고 처리, 사용자 관리, 장소 데이터, 통계 - #83

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/