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/domain/admin/service/task_trigger_test.go

@@ -0,0 +1,32 @@
+// Copyright 2026 Arctel.net
+// SPDX-License-Identifier: Apache-2.0
+
+package service
+
+import (
+	"Wavelet/core/contracts"
+	"testing"
+)
+
+func TestNormalizeTaskTrigger(t *testing.T) {
+	tests := []struct {
+		in   string
+		want string
+	}{
+		{in: contracts.TaskTriggerManual, want: contracts.TaskTriggerManual},
+		{in: contracts.TaskTriggerSystem, want: contracts.TaskTriggerSystem},
+		{in: contracts.TaskTriggerRetry, want: contracts.TaskTriggerRetry},
+		{in: contracts.TaskTriggerSchedule, want: contracts.TaskTriggerSchedule},
+		{in: "http", want: contracts.TaskTriggerSystem},
+		{in: "inproc_cron", want: contracts.TaskTriggerSchedule},
+		{in: "cron", want: contracts.TaskTriggerSchedule},
+		{in: "", want: contracts.TaskTriggerSystem},
+		{in: "custom", want: "custom"},
+	}
+	for _, tt := range tests {
+		got := normalizeTaskTrigger(tt.in)
+		if got != tt.want {
+			t.Errorf("normalizeTaskTrigger(%q) = %q, want %q", tt.in, got, tt.want)
+		}
+	}
+}