chore: Implement a graceful shutdown sequence for the Node server

[AlAfiz]

Description

When we deploy updates to the backend, the server process is killed abruptly.
If a user's API request is currently processing, it will fail suddenly without a proper response.
We need to implement a "Graceful Shutdown" script that catches termination signals (SIGINT, SIGTERM).
This script should stop accepting new requests and finish processing active requests before shutting down completely.

Requirements

• Add event listeners for process.on('SIGTERM') and process.on('SIGINT').
• Inside the listener, call server.close() to stop accepting new HTTP connections.
• Add a console.log("Gracefully shutting down TradeFlow API...").
• Use process.exit(0) after the server successfully closes all active connections.