Reactivate Subscription

Resume a cancel-pending subscription.

On cancel we delete the Mollie subscription but keep access until period end; the SEPA mandate normally stays valid, so we resume by creating a fresh subscription on it (no payment) starting at the paid-through date — never a double charge. If the mandate has lapsed/been revoked, fall back to a first payment to re-authorise (returns a checkout URL for the frontend to redirect).