chore: 合并为单仓库，统一部署流程

- 根目录加入 docker-compose、Caddyfile、文档等原本不在仓库里的文件
- 前后端两份 GitHub Actions 合并成根目录的一个 deploy workflow：
  构建前端上传到 build/，再在服务器上 git pull 并重建容器

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

client/.github/workflows/deploy.yaml

@@ -1,44 +0,0 @@
-name: Deploy
-
-on:
-  push:
-    branches:
-      - main
-
-permissions:
-  contents: read
-
-jobs:
-  deploy:
-    runs-on: ubuntu-latest
-    strategy:
-      fail-fast: false
-      matrix:
-        include:
-          - name: debian
-            build_command: build
-            remote_port: 22
-          - name: school
-            build_command: build:staging
-            remote_port: 8822
-    steps:
-      - uses: actions/checkout@v4
-
-      - uses: actions/setup-node@v4
-        with:
-          node-version: 24
-          cache: npm
-      - run: npm install
-      - run: npm run ${{ matrix.build_command }}
-        env:
-          CI: false
-
-      - uses: easingthemes/ssh-deploy@main
-        with:
-          SSH_PRIVATE_KEY: ${{ secrets.KEY }}
-          REMOTE_HOST: ${{ secrets.HOST }}
-          REMOTE_PORT: ${{ matrix.remote_port }}
-          ARGS: "-avzr --delete"
-          SOURCE: dist/
-          REMOTE_USER: root
-          TARGET: /root/WebPreview/build