#4, добавила роутер /сategories

app/categories.js

+const express = require("express");
+const Category = require('../models/Category');
+
+const createRouter = () => {
+  const router = express.Router();
+
+  router.get("/", async (req, res) => {
+    try {
+      res.send(await Category.find());
+    } catch (e) {
+      res.status(500).send(e)
+    }
+  });
+
+  return router;
+};
+
+
+module.exports = createRouter;