Open the Stripe Billing Portal for the caller's organization.

Org-admin only, via org_api's require_org_admin — billing is an admin surface, and there must be exactly one definition of "org admin."

Every lookup below is keyed on admin["organization_id"], never on the caller's uid. The user-keyed billing query is WHERE ou.user_id = %s LIMIT 1 with no ORDER BY, so for a user with two membership rows it can return a different org than the one require_org_admin authorised — which would mean provisioning one org and opening the portal on another org's Stripe customer.