Cancel the org's Pro subscription at the end of the current billing period — the server-side equivalent of cancelling through the Stripe Customer Portal. The subscription (and its entitlements) stays active until the period ends, when Stripe terminates it and the org returns to the Base plan; no refund or proration is issued. Idempotent: a subscription already pending cancellation returns no_op. Requires an active Pro subscription with a provisioned Stripe subscription. The optional body carries the cancel survey (feedback from Stripe's fixed vocabulary, free-text comment), forwarded to Stripe as cancellation_details so the admin Revenue page's Pending Cancellations reason column is populated for in-app cancels the same way it is for Customer Portal cancels.