Get associated app

## Get the app of a payment request Returns the app (sale terminal) associated with the payment request `:id`, including its category. Use it to know where a payment link or invoice is being collected. ### Authentication - Send the merchant API key in **`x-api-key`** (alias `x-account-api-key`). - Requires **`Merchant.payment_link.read`**. - Optionally scope the key with **`x-application-account-id`** (alias `x-account-id`). ### Path params - `id` — the payment request id. ### Behavior - The app is looked up by the caller's store **and** the payment request id. If the payment request has no app (or belongs to another store) the API responds **400 Bad Request** (`App for payment request <id> not found.`). There is **no request body**. --- **Notas:** Handler: paymentRequestAppsService.getAppByPaymentRequestId. Accepts the deprecated alias GET /v1/payment-links/:id/app. The response is formatted by appFormatterService.transformAppForResponse with the Lnurlp relation omitted.