Store events and create deliveries in the delivery engine (closes #544)

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

internal/delivery/inflight.go

@@ -61,7 +61,7 @@ func (s *inflightSet) retain(deliveryID string) {
 
 // retainIdle takes the first reference on a delivery, and reports
 // whether it got it. It fails when the engine already owns the
-// delivery, which is what makes two claimants — Notify and the
+// delivery, which is what makes two claimants — notify and the
 // recovery routine, say — mutually exclusive rather than merely
 // atomic.
 func (s *inflightSet) retainIdle(deliveryID string) bool {