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_integration_test.go

@@ -492,7 +492,7 @@ func TestWorkerLifecycle_StartStop(t *testing.T) {
 	)
 	task.TargetType = database.TargetTypeLog
 
-	s.Engine.Notify([]delivery.Task{task})
+	s.Engine.ExportNotify([]delivery.Task{task})
 
 	iWaitForDelivered(t, s.WebhookDB, d.ID)
 
@@ -1452,7 +1452,7 @@ func TestDeliverHTTP_NoURL(t *testing.T) {
 	iAssertResultFailed(t, db, del.ID)
 }
 
-// --- Notify batching ---
+// --- notify batching ---
 
 func TestNotify_MultipleTasks(t *testing.T) {
 	t.Parallel()
@@ -1467,7 +1467,7 @@ func TestNotify_MultipleTasks(t *testing.T) {
 		}
 	}
 
-	e.Notify(tasks)
+	e.ExportNotify(tasks)
 
 	for i := range 5 {
 		select {