Mint a one-time billing-portal link

Returns a ONE-TIME URL to the Paddle customer portal for the calling key: change plan, update the card, download every invoice, cancel.

TREAT THE URL AS A CREDENTIAL. Whoever opens it is treated as the account holder — there is no second factor behind it. Do not cache it, do not log it, do not put it in a shared inbox or a chat channel, and do not hand the same one to two people. It expires; request a fresh one instead of storing it. (Paddle documents that portal sessions expire but publishes no duration, so this API does not invent one — there is deliberately no expires_at field rather than a guessed one.)

GET and POST are identical. Consumes no documents; it is subject to the same per-second pace limit as everything else on the key.

A key with no subscription behind it — a free key, or a tier granted by hand — gets 409 no_subscription. Every receipt email Paddle sends also carries a portal link, which is the path for a customer who cannot reach this API.