Self-service billing portal link

Cost: 0 credits. Returns a short-lived Stripe billing portal URL for this key's subscription: update the card, download invoices, see what the next charge will be.

THE API KEY IS THE IDENTITY. No website account or separate login is needed: the key that holds the subscription opens its portal, so a customer who subscribed through the API can change a failing card, pull invoices or cancel without contacting us.

The returned URL is a BEARER link: anyone holding it can manage the subscription until it expires. It is deliberately never logged, and callers should not paste it anywhere.