Amend or cancel a previously created BACS direct debit paper mandate.

The requested operation is validated against the mandate's current status and version, the pending intent is persisted with an optimistic-lock version bump, and the operation is enqueued for the asynchronous submission worker to forward to Form3. The endpoint does not call Form3 directly.

State rules:

• amend is allowed only from CREATED, SUBMITTED, or CONFIRMED, and moves the mandate to AMEND_PENDING.
• cancel is allowed only from SUBMITTED or CONFIRMED, and moves the mandate to CANCEL_PENDING.
• An invalid source state returns 409 Conflict.

Optimistic concurrency: the caller sends the version it last observed. If it does not match the stored version, or the mandate is modified concurrently, the request is rejected with 409 Conflict. On success the version is incremented by 1.

This endpoint generates a BACS paper mandate status changed event with status = AMEND_PENDING (amend) or CANCEL_PENDING (cancel).