fix: 使用 Vercel Functions API 配置

- 移除 builds 和 routes 配置
- 使用 functions 配置指定运行时
- 添加 buildCommand 确保构建

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

vercel.json

@@ -1,15 +1,9 @@
 {
   "version": 2,
-  "builds": [
-    {
-      "src": "package.json",
-      "use": "@vercel/node"
+  "buildCommand": "pnpm run build",
+  "functions": {
+    "api/*.ts": {
+      "runtime": "@vercel/node@3.0.0"
     }
-  ],
-  "routes": [
-    {
-      "src": "/(.*)",
-      "dest": "/dist/api/index.js"
-    }
-  ]
+  }
 }