Amend a BACS direct debit mandate.

Registers a change to a mandate's debtor and/or beneficiary details, applied asynchronously through the BACS scheme.

Notes:

• The amend is persisted with status = AMEND_PENDING, and enqueued for the asynchronous submission worker, which submits it to Form3 through the PSP gateway.

• Allowed only when the mandate is in CREATED, SUBMITTED, or CONFIRMED; any other state is rejected with a 409 Conflict (EBACS0103).

• version is a Form3 optimistic-concurrency passthrough that must equal the stored version; a mismatch is rejected with a 409 Conflict (EBACS0104). This call does not increment this field.

• This endpoint moves the mandate to status = AMEND_PENDING and generates a BACS mandate status changed event. Subsequent status transitions generate further events (for example AMENDED, FAILED).