Reactivate the org's Pro subscription by removing a pending cancellation before the current billing period ends — the server-side equivalent of clicking "Do not cancel" in the Stripe Customer Portal. The subscription resumes renewing at the period end; nothing is charged now. Idempotent: a subscription with no pending cancellation returns no_op. Requires an active Pro subscription with a provisioned Stripe subscription — once the period has ended and the subscription is fully canceled it can no longer be reactivated (returns HTTP 403; subscribe again via upgrade instead).