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

package.json

@@ -2,7 +2,6 @@
   "name": "jaryo-file-manager",
   "version": "2.0.0",
   "description": "자료실 파일 관리 시스템 - Vercel Serverless",
-  "type": "module",
   "scripts": {
     "dev": "vercel dev",
     "build": "echo 'Build complete'",