Cancel the subscription at the end of the paid period

The caller's own account. Schedules the subscription to end at current_period_end — the customer keeps the paid plan for the period they paid for, and the account only reads free once Stripe's customer.subscription.deleted lands at that boundary.

The response reports what STRIPE accepted. plan on the billing summary is written by the customer.subscription.* webhook a moment later, so a summary fetched in the same breath may still report the old plan — render the value returned here and re-fetch the summary shortly after, rather than treating the lag as a failure.