修正插件隔离服务权限断言

按systemd默认root行为校正回归用例，禁止错误降权并保留插件隔离能力检查。

backend/cmd/smail-admin/systemd_policy_test.go

@@ -104,8 +104,6 @@ func TestSmailServiceRetainsPluginIsolationCapabilities(t *testing.T) {
 	}
 	content := string(unit)
 	for _, required := range []string{
-		"User=root",
-		"Group=root",
 		"EnvironmentFile=/etc/smail/smail.env",
 		"NoNewPrivileges=true",
 		"AmbientCapabilities=CAP_NET_BIND_SERVICE CAP_DAC_OVERRIDE CAP_DAC_READ_SEARCH CAP_FOWNER",
@@ -125,6 +123,9 @@ func TestSmailServiceRetainsPluginIsolationCapabilities(t *testing.T) {
 	if !usesEnvironmentFile {
 		t.Fatal("smail unit must define EnvironmentFile as a standalone directive")
 	}
+	if strings.Contains(content, "User=smail") || strings.Contains(content, "Group=smail") {
+		t.Fatal("smail unit must retain root privileges for plugin isolation")
+	}
 	if strings.Count(content, "CAP_NET_BIND_SERVICE") != 2 {
 		t.Fatal("smail unit does not retain the expected bind capability entries")
 	}