Delete a customer

The accounting platform books and sends the credit note inside the request, and the answer is the booked credit note, 201. A full credit takes the whole invoice back and is refused once anything was credited (`INVOICE_ALREADY_CREDITED`); a partial credit names lines of the invoice, each by no more than is left on it (`VALIDATION_FAILED` with `AMOUNT_EXCEEDS_BILLED` under the line). Amounts are before VAT. **Sending it again is safe**: the same `external_id` is the same credit note, whatever its body says the second time. Booked, it is answered as it stands, 200, and nothing is booked twice; not booked, that attempt is taken up again under its own date and lines. **A credit note that booked and could not be delivered is a success** whose `send_error` says why. An attempt the platform gave no definite answer to holds every other credit on the invoice back (`CREDIT_ATTEMPT_UNRESOLVED`, 409) until the request that made it is sent again. Metered against the `platform_write` bucket as well as `default`.