A signed link to your prorated upgrade page

Cost: 0 credits. For the signed-in account page (2026-09-28): /v1/billing/upgrade identifies its reader by X-API-Key or by an emailed HMAC token, and a browser navigating to a page sends neither — so this mints the same 30-minute token for the caller's own customer id and returns the page URL. Without a subscription there is nothing to prorate and the answer is the ordinary pricing page. Like the portal URL, treat the link as a bearer credential for the subscription.