Customer Billing Portal context.

Thin facade over Accrue.BillingPortal.Session.create/1. The portal itself is hosted by Stripe — Accrue's job is wrapping session creation, masking the bearer-credential URL in Inspect, and providing the :configuration passthrough for hosts that pre-create a bpc_* configuration in the Stripe Dashboard.

See guides/portal_configuration_checklist.md for the three Dashboard toggles every host app should enable to defend against the "cancel-without-dunning" footgun (Pitfall 6).