CancelPaidSubscription

Validates POST /v1/billing/cancel (D-32 + D-42). Body: cancellation_reason: required, one of CancellationReason enum values. retention_pitch_shown_at: nullable ISO8601 — when the SPA last displayed the retention copy before submit (lifecycle drip target). Headers: Idempotency-Key: required UUIDv7 (D-32). Forwarded to the provider so caller-side retries collapse on Mollie's idempotency layer. Authorization: Owner-only (D-42). See {@see PlanChangePreviewRequest} for the inline-pivot rationale; same shape applies here.