Bundled Chimeway.Notifier implementation for Accrue dunning notifications.
Resolves Accrue domain models (Subscription → Customer) to produce
email-channel recipient maps. Implements only the 4 required callbacks
plus channels/2 (email-only) and orchestration/2 (:immediate).
workflow/2 is intentionally omitted for the v1.40 email-only :immediate
path — see Accrue.Integrations.Chimeway moduledoc for rationale.