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/export_test.go

@@ -158,6 +158,11 @@ func (e *Engine) ExportUpdateDeliveryStatus(
 	return e.updateDeliveryStatus(webhookDB, d, "", status)
 }
 
+// ExportNotify exposes notify.
+func (e *Engine) ExportNotify(tasks []Task) {
+	e.notify(tasks)
+}
+
 // ExportProcessNewTask exposes processNewTask.
 func (e *Engine) ExportProcessNewTask(
 	ctx context.Context, task *Task,