saas-dr: CancelSaasResponseAction to revoke a proposed action

[dcoln25-writer]

Phase 2 follow-up.

A proposer cannot withdraw a response action once they propose it; the only way to leave PROPOSED is via Approve / Execute. Operators need a CANCEL path for accidentally proposed actions or after-the-fact context changes.

Scope

• Add CancelSaasResponseAction(id, reason) RPC.
• Allow cancellation from PROPOSED or APPROVED states only (NOT after EXECUTING / SUCCEEDED / FAILED).
• Set status to CANCELLED, persist cancelled_by_user_id and cancelled_at, emit a timeline event and audit log row.
• Update the state machine in executeSaasResponseAction to reject CANCELLED actions explicitly.
• Frontend: cancel button on the proposed-actions list, with a reason prompt.

Acceptance

• New columns in saas_response_actions (with migration).
• Tests cover the allowed and disallowed transitions and tenant isolation.
• Cancellation by a third user (neither proposer nor approver) is allowed if they have the right role.