Satyam8589/Backend-Vyapar-Sathi ? reverse-engineered prompt

Build me the backend for a multi store retail POS and inventory app called Vyapar Sathi.

It should let users sign in securely, create and manage stores, and keep every store’s data fully separate. I need APIs for products, stock in and stock out, sales, purchases, purchase orders, goods received notes, sellers, customers, expenses, and staff roles. The POS flow should support barcode scanning, carts, checkout, and different payment types like cash, UPI, QR, and split payments. When a sale or purchase happens, stock and records should update right away.

Please also include real time updates for billing and stock alerts, background jobs for notifications and reports, and a proxy route for an AI copilot service. Add file upload support for product images, PDF invoice generation, email notifications, and basic testing for the main workflows. Use a clean Node and MongoDB setup, and look up current docs online if you need to.