feat: 大文件分片级断点续传（(b) 方案：抽 upload_parts + 会话落 SQLite） - #7

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/