Create a Stripe portal URL for account billing changes

Create a billing session and return {portal_url}. Minting the URL costs nothing quota-wise; Stripe applies any card, plan, or cancellation change selected by the human. Call this for an existing account, then relay portal_url verbatim instead of opening it for the user. Recover from invalid_key by replacing the API key and from billing_unavailable or rate_limited by waiting retry_after_sec before one retry.