fix: add @types/cors and fix jwt expiresIn type

Co-Authored-By: Claude Haiku 4.5 <noreply@anthropic.com>

backend/package.json

@@ -29,6 +29,7 @@
   "devDependencies": {
     "@types/express": "^4.17.20",
     "@types/node": "^20.8.0",
+    "@types/cors": "^2.8.17",
     "@types/jsonwebtoken": "^9.0.4",
     "@types/bcryptjs": "^2.4.2",
     "typescript": "^5.2.2",