chore: Unify backend express project into ESM (#281)

backend/routes/index.js

@@ -1,9 +1,10 @@
-const express = require('express');
+import express from 'express';
 const router = express.Router();
 
-// "/" GET
-router.get('/', (req, res) => {
-  res.send('This is UnitTCMS API server');
-});
+export default function () {
+  router.get('/', (req, res) => {
+    res.send('This is UnitTCMS API server');
+  });
 
-module.exports = router;
+  return router;
+}