Create a Stripe checkout URL for a listed plan

Create checkout and return {checkout_url, expires_at}. Minting the URL costs nothing quota-wise; the selected paid plan changes billing only after Stripe confirms payment. Call GET /v1/billing first, send one Idempotency-Key per offer, and relay checkout_url verbatim to the human principal. Recover from plan_unknown by choosing a returned plan id, from already_subscribed with POST /v1/billing/portal, and from billing_unavailable by waiting retry_after_sec before one retry.