Cancel a BACS direct debit paper mandate.

Cancel of an existing paper mandate, applied asynchronously through the BACS scheme.

Notes:

• The cancel is persisted with status = CANCEL_PENDING under the payer account, and enqueued for the asynchronous submission worker, which submits it to Form3 through the PSP gateway.

• Allowed only when the mandate is in 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 it.

• The optional status_reason records why the mandate is being cancelled; it is persisted on the mandate and forwarded on the generated event.

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