fix: 指定 Node.js 18.x 版本

- 添加 engines 字段指定 Node.js 18.x
- 修复 Vercel 部署版本不匹配错误

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

package.json

@@ -5,6 +5,9 @@
   "author": "",
   "private": true,
   "license": "UNLICENSED",
+  "engines": {
+    "node": "18.x"
+  },
   "scripts": {
     "build": "npx nest build",
     "format": "prettier --write \"src/**/*.ts\" \"test/**/*.ts\"",