Create a BACS direct debit paper mandate from the details of a signed paper mandate form.

The mandate is imported into Form3 through the PSP gateway, persisted with status CREATED under the account indicated in the account token (role PAYER), and enqueued for the asynchronous submission worker. The endpoint does not call Form3 directly.

Notes:

• The operation is idempotent via the Idempotency-key header. Repeating the same key replays the original mandate; reusing the same reference with a different key is rejected with a 409 Conflict.

• This endpoint creates the mandate with status = CREATED, which does not generate an event. Subsequent status transitions generate a BACS paper mandate status changed event (for example: SUBMITTED, CONFIRMED, CANCELLED, FAILED).