AccrueAdmin — Phoenix LiveView admin UI for Accrue billing.
The admin package is released in lockstep-major with :accrue, from the
same monorepo (sibling mix project, non-umbrella; see D-42 / D-43). At
Wave 0 this module is a namespace anchor; the concrete LiveView dashboard
(customers, subscriptions, invoices, charges, coupons, webhook events,
activity feed) lands in Phase 7.