Get checkout payload

## Get invoice checkout payload Returns the **full checkout representation** (`InvoicePosDto`) of the invoice `:id`: current status, resolved payment method, crypto amount and payment destination, exchange rates and the hosted invoice URL. This is what a checkout page needs to render the payment. ### Auth `x-api-key` header with **`Merchant.invoice.read`**. Also accepts JWT user, POS JWT and limited JWT tokens. ### Path params - `id` (required): the invoice id. There is **no request body**. --- **Notas:** Handler: statusService.getStatus({ invoiceId, complete: true }). Same handler as GET /invoice/:id/status but returns the complete formatted payload. The DTO keeps deprecated aliases (rate, satsDue, cryptoCode, btcAddress, nationalRate, invoiceWallet, btcDue) — prefer exchangeRate, cryptoAmount, paymentType, paymentDestination, usdExchangeRate.