WhatsApp Multi-Session Node.js API

I already have a working Node.js endpoint that creates a WhatsApp QR code and can send messages once the code is scanned. Right now it supports only a single number because the session information is not saved anywhere. I need the logic extended so every mobile number that scans the QR code gets its own persistent session and the API can reuse that session whenever a message is sent to or from that number.

The revised code must be able to handle multiple sessions at the same time without interrupting each other, and it should expose clear methods to:
• create / refresh a QR code for a new number
• store the session data when the code is scanned
• retrieve the correct session seamlessly when a message needs to be pushed

I’m open to your recommendation on where the session data lives—database, file system, or another approach that makes sense for production—but the solution needs to remain simple to deploy alongside the existing Node.js stack. Please keep external dependencies minimal and document any setup steps, especially if a library such as whatsapp-web.js, Baileys, or puppeteer-based tooling is involved.

Deliverables will be the updated API source, concise setup instructions, and a short README that shows example requests for onboarding a number and sending a follow-up message with its saved session.