Fix Vercel serverless deployment: optimize for fast loading

- Convert Express app to Vercel serverless function
- Add missing /api/files/public endpoint
- Optimize static file routing with proper caching
- Remove unnecessary dependencies for faster cold starts
- Add comprehensive debugging and error handling
- Improve API response times and user experience

package.json

@@ -1,28 +1,21 @@
 {
   "name": "jaryo-file-manager",
-  "version": "1.0.0",
-  "description": "자료실 파일 관리 시스템",
-  "main": "server.js",
+  "version": "2.0.0",
+  "description": "자료실 파일 관리 시스템 - Vercel Serverless",
+  "type": "module",
   "scripts": {
-    "start": "node server.js",
-    "dev": "nodemon server.js",
-    "init-db": "node scripts/init-database.js"
-  },
-  "dependencies": {
-    "express": "^4.18.2",
-    "sqlite3": "^5.1.6",
-    "cors": "^2.8.5",
-    "multer": "^1.4.5-lts.1",
-    "path": "^0.12.7",
-    "fs": "^0.0.1-security",
-    "bcrypt": "^5.1.1",
-    "express-session": "^1.17.3",
-    "uuid": "^9.0.1"
+    "dev": "vercel dev",
+    "build": "echo 'Build complete'",
+    "start": "vercel dev"
   },
+  "dependencies": {},
   "devDependencies": {
-    "nodemon": "^3.0.1"
+    "vercel": "^32.0.0"
   },
-  "keywords": ["file-manager", "sqlite", "express", "admin"],
+  "keywords": ["file-manager", "vercel", "serverless", "admin"],
   "author": "Claude Code",
-  "license": "MIT"
+  "license": "MIT",
+  "engines": {
+    "node": ">=18.0.0"
+  }
 }