jotiram75/MeshPay-UPI ? reverse-engineered prompt

Build me a Spring Boot demo app that shows a fake UPI payment moving through a Bluetooth style mesh when there’s no internet.

I want a dark web dashboard where I can enter a sender, receiver, amount, and PIN, then inject a payment into the mesh, run a few gossip rounds, and finally have a bridge phone upload it to the backend so the money gets settled. The app should show the packet hops, which devices are holding it, current account balances, and a simple transaction ledger.

Please make the backend handle encrypted payment packets, reject tampered or replayed ones, and make sure the same payment can only settle once even if multiple bridge devices upload it at the same time. Include a couple of simple tests for the duplicate upload case.

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