Give 60 days' notice of cancellation.

Gives the Customer Agreement's 60-day notice of cancellation. Records the end date (cancelsAt, 60 days out) and sets the same date on the Stripe subscription; billing, portal access and patient checkout continue until then, when the subscription ends and the clinic is deactivated. Idempotent: a second call returns the dates already recorded. The notice can be withdrawn before the date with POST /api/v1/billing/cancel/withdraw. Write-scoped.