sequenceDiagram
  box Tu sistema de caja
      participant CAJA as Caja registradora<br/>POS / TPV
      participant BE as Backend del comercio
      participant DB as Base de datos
  end
  box Yappy
      participant API as Yappy API<br/>session + qr + transaction
      participant APP as Yappy App<br/>del cliente
  end

  CAJA->>BE: 1. Abre sesión
  BE->>API: 2. POST /session/device
  API-->>BE: 3. token
  BE->>API: 4. POST /qr/generate
  API-->>BE: 5. transactionId + hash
  BE-->>CAJA: 6. Renderiza QR
  APP->>CAJA: 7. Cliente escanea y paga
  BE->>API: 8. Polling GET /transaction/:id
  BE->>DB: 9. Guarda