Update server.js

Modify package.json description and engines

Integrate OpenAI API in chat endpoint

Refactor auth middleware and healthcheck route

Set up Express server with middleware and endpoints

Refactor package.json with dependencies and scripts

Delete folder cleanup

Delete README.md for Threadwise backend

Rename README.md to threadwise-backend/README.md

Create .gitkeep

Add full Threadwise backend implementation and configuration files.

Initialize repository with README file