fix(tasks): canonicalize triggered_by so execution labels resolve

Unknown values such as http and inproc_cron made the admin UI call
t(undefined). Dispatch sites now write system/manual/retry/schedule,
the list API maps legacy rows, and the table skips missing i18n keys.

backend/plugins/drivers/driver_inproc_worker/executor.go

@@ -76,7 +76,7 @@ func (q *InprocQueue) Enqueue(ctx context.Context, taskType string, payload []by
 	}
 
 	if source == "" {
-		source = "manual"
+		source = contracts.TaskTriggerManual
 	}
 	idType := td.Type
 	if idType == "" {