Manage subscription

Creates a Stripe customer-portal session so the org's admin can self-serve update card / view invoices / cancel subscription. authz: allowed_org_types=[provider], min_org_role=administrator (same gate as create-checkout-session — cancelling/updating-card has the same financial weight as subscribing). Returns 404 use_create_checkout_session if the org has no Stripe customer. POST /billing/create-checkout-session/v1 first. | (no body) -> (CreateCustomerPortalSessionRes)