issue with server middleware when deploying nuxt-app on heroku

I'm running into an issue to deploy a nuxt-app with some serverMiddleware code on Heroku.

this is my middleware code which just launch an express app:

api/index.js

var express = require("express");
var nodemailer = require("nodemailer");
var app = express();

app.use(express.json());
app.use(
  express.urlencoded({
    extended: true
  })
);

app.get("/", (req, res) => {
  res.send("hello world");
});

app.post("/contact", async (req, res) => {
  // let testAccount = await nodemailer.createTestAccount();

  // create reusable transporter object using the default SMTP transport
  let transporter = nodemailer.createTransport({
    host: "mail.exemple.net",
    port: 587,
    secure: false,
    auth: {
      user: process.env.EMAIL_USERNAME,
      pass: process.env.EMAIL_PASSWORD
    }
  });

  // send mail with defined transport object
  let info = await transporter.sendMail({
    from: req.body.email,
    to: "contact@email.com",
    subject: "new contact",
    text: req.body.message
  });

  res.end();
});

app.listen(process.env.PORT);

module.exports = {
  path: "/api",
  handler: app
};

then i registered this file into nuxt.config.js:

serverMiddleware: ["~/api/index"], 

the api works perfectly in dev mode (locally).

I first deployed on heroku with the nuxt target setting set to static, the static content was working perfectly but none of the serverMiddleware code was working (that make senses ^^).

then i switched to target: 'server' but now when i deploy i get this message from heroku:

✖ Nuxt Fatal Error
Error: Address `0.0.0.0:41828` is already in use

i'm sure that i'm missing something but can't find the solution.

just to complete my post i added this line into the package.json file:

"heroku-postbuild": "nuxt build"

and also a Procfile at the root of my app which contain:

web: nuxt start

and finally these are my heroku:config settings:

heroku config:set NPM_CONFIG_PRODUCTION=false
heroku config:set HOST=0.0.0.0
heroku config:set NODE_ENV=production

Any advice on the correct way to deploy much appreciated.