feat: add request ID middleware for request tracing

[shivv23]

Add request ID middleware to track requests across services.

Changes:

• app/middleware.py: Add RequestIDMiddleware class
• app/main.py: Register the middleware

The middleware generates a UUID for each request if not provided,
and adds X-Request-ID header to responses.

[ivantha]

This PR's head branch is behind main. Please rebase onto the latest main (e.g. git fetch origin && git rebase origin/main) and force-push to make it mergeable.

[shivv23]

Thanks @ivantha! The branch has been rebased onto main and is now mergeable.