POST /v1/bookings/{id}/payment-plan/cancel

Cancels an active payment plan. Returns 409 if the plan is already in a terminal state (cancelled / completed / failed) — idempotent retry is the SDK caller's responsibility once the plan is done.

Uses the model's cancel() helper (Phase 155 T8) which:

• wraps in a transaction
• flips pending/due/retrying items to cancelled
• busts the dashboard summary cache We do NOT re-send the customer/admin emails here — that path is reserved for portal-initiated cancellations. API cancels are operator-initiated; the operator's own system is the authority.