Fix serverless functions: simplify API structure

- Replace complex Express-style handler with simple module.exports
- Add separate API endpoints for better organization
- Simplify vercel.json routing configuration
- Remove ES modules to use CommonJS for Vercel compatibility
- Add dedicated test API endpoint for debugging

api/test.js

@@ -0,0 +1,21 @@
+// 테스트용 간단한 API
+module.exports = (req, res) => {
+    // CORS 헤더 설정
+    res.setHeader('Access-Control-Allow-Origin', '*');
+    res.setHeader('Access-Control-Allow-Methods', 'GET, POST, OPTIONS');
+    res.setHeader('Access-Control-Allow-Headers', 'Content-Type');
+    
+    if (req.method === 'OPTIONS') {
+        res.status(200).end();
+        return;
+    }
+
+    // 간단한 응답
+    res.status(200).json({
+        success: true,
+        message: 'API가 정상 작동합니다!',
+        timestamp: new Date().toISOString(),
+        url: req.url,
+        method: req.method
+    });
+};