Open the billing portal for a workspace

Returns a Stripe customer portal URL. The billing subject is the workspace's OWNER on every lane, and a session-lane caller who is not the owner is refused 403 — a portal session covers the owner's whole Stripe account. See the checkout route's note.

Requires the billing:write scope. Accepts all three lanes: a session JWT (bounded by the caller's role on this workspace) or an il_* machine credential whose own scope set, workspace set and validity window cover it — both ceilings apply and neither can raise the other, so a credential carrying billing:write is still refused where the role does not permit it.

A credential with no scope record resolves to LEGACY_DEFAULT, which does NOT include billing:write — mint a key with it explicitly (POST /v2/workspaces/{id}/keys).