Compatibility lifecycle seam for adopter-side event composition.

The callback shape remains handle_event(Ecto.Multi.t(), map()) :: Ecto.Multi.t(). For one-click unsubscribe, Mailglass calls handle_event(Ecto.Multi.new(), attrs) only after the primary event and suppression convergence commits, then runs the returned Multi as a separate, best-effort transaction. A lifecycle failure is logged and cannot roll back or change the already-successful unsubscribe response.