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

The receiver, resubmit and replay handlers now read and check 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/engine_test.go

@@ -218,7 +218,7 @@ func TestNotify_NonBlocking(t *testing.T) {
 	done := make(chan struct{})
 
 	go func() {
-		e.Notify([]delivery.Task{
+		e.ExportNotify([]delivery.Task{
 			{DeliveryID: "overflow-1"},
 			{DeliveryID: "overflow-2"},
 		})
@@ -229,7 +229,7 @@ func TestNotify_NonBlocking(t *testing.T) {
 	case <-done:
 	case <-time.After(2 * time.Second):
 		t.Fatal(
-			"Notify blocked when delivery channel was full",
+			"notify blocked when delivery channel was full",
 		)
 	}
 }