feat(server): 新增云端缓存与同步服务端骨架

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-openagent)

Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

server/README.md

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+# Magnet Cloud Cache Server
+
+共享缓存池后端最小骨架：
+
+- Fastify 4
+- MySQL 5.7+/8.0+
+- AES-256-GCM 数据库存储加密
+- 共享线程缓存 / 范围缓存 / 页缓存
+- 为未来账号体系和私有保险箱预留表结构
+
+## 快速启动
+
+1. 复制 `.env.example` 为 `.env`
+2. 填好 MySQL 和加密密钥
+3. 安装依赖：`npm install`
+4. 启动：`npm start`
+
+## 已实现接口
+
+- `GET /health`
+- `GET /ready`
+- `POST /api/shared-cache/threads/lookup`
+- `POST /api/shared-cache/threads/upsert`
+- `POST /api/shared-cache/coverages/lookup`
+- `POST /api/shared-cache/coverages/upsert`
+- `POST /api/shared-cache/pages/lookup`
+- `POST /api/shared-cache/pages/upsert`