Cancel Subscription Route

Cancel the caller's subscription, self-serve.

Deliberately NOT named /billing/cancel: that path is already the Checkout-abandoned landing page, and one word meaning two opposite things is how a customer ends up on the wrong one.

The mode is chosen from the subscription's own status rather than asked of the customer, because only one answer is ever right:

active / trialing -> cancel at period end. They paid for this period, they keep it, the key drops to Free when it runs out. past_due / unpaid / incomplete -> cancel now. There is no paid period left to keep, and at_period_end would leave Stripe retrying the failed payment, which is the thing ticket #271 asked us to stop.

An immediate cancel files a support ticket for the open invoice. We do not void it here on purpose: that is a money decision.