The receiver, resubmit and replay handlers now parse the request and
make one call. Engine.StoreEvent stores an event with a pending
delivery for each active target and the totals in one transaction,
then queues them; Engine.CreateReplay does the same for one replay
delivery. The handlers take *delivery.Engine, so the Notifier
interface and its bridge are gone, and the engine's notify is no
longer exported. Archives and CircuitBreakers stay because the handler
tests fake them. The tests that read a queued task moved to the
engine's tests; the handler tests use an engine that is never started.
Model: opus-5-5
