fix: add module type to package.json and remove unused import in server.js

backend/package.json

@@ -1,6 +1,7 @@
 {
   "name": "backend",
   "version": "1.0.0",
+  "type": "module",
   "main": "index.js",
   "scripts": {
     "test": "echo \"Error: no test specified\" && exit 1",
@@ -18,4 +19,4 @@
     "jose": "^6.0.12",
     "mysql2": "^3.14.3"
   }
-}
+}