Front-End & Back-End Integration

I have a working front end written in plain HTML and JavaScript and a lightweight back end that also runs in JavaScript. Right now they operate in isolation. I need the two layers wired together so every piece of data the browser sends is correctly received, checked, and acted on by the server. Your main tasks will be: • Create the necessary routes / handlers so the back end can accept incoming requests from the front end. • Add solid data validation on every payload before it is stored or forwarded. • Integrate two external APIs (one for analytics, one for reference data) and return the responses to the front end in the required format. I will hand over the existing codebases, sample payloads, and the API docs as soon as we start. Please keep the structure clean and well-commented so I can follow along later. Once everything is connected and tested, a brief walkthrough or README explaining how to run and extend the solution will complete the job.