refactor: 拆分 system.rs 大文件，将 email_templates/proxy_errors/system_config 下沉到 shared 层

- 删除 1666 行的 admin/system/shared/system.rs，按职责拆分到独立模块
- 新增 handlers/shared/email_templates.rs 和 system_config_values.rs 存放跨层共用的模板/配置工具函数
- 新增 admin/system/shared/email_templates.rs 存放 admin 专用的模板操作逻辑
- 新增 admin/shared/proxy_errors.rs 存放 build_proxy_error_response
- 清理 public/system_modules_helpers 中不属于 public 层的导出
- 新增架构测试守护模块归属边界

apps/aether-gateway/src/handlers/admin/features/video_tasks/routes.rs

@@ -1,6 +1,8 @@
 use crate::async_task;
 use crate::control::GatewayPublicRequestContext;
-use crate::handlers::admin::shared::{attach_admin_audit_response, query_param_value};
+use crate::handlers::admin::shared::{
+    attach_admin_audit_response, build_proxy_error_response, query_param_value,
+};
 use crate::{AppState, GatewayError};
 use aether_data_contracts::repository::video_tasks::{VideoTaskQueryFilter, VideoTaskStatus};
 use axum::{
@@ -11,7 +13,6 @@ use axum::{
 };
 use serde_json::json;
 
-use super::super::super::auth::build_proxy_error_response;
 use super::builders::{
     admin_video_task_detail_id_from_path, admin_video_task_nested_id_from_path,
     admin_video_task_status_name, admin_video_task_timestamp, build_admin_video_task_list_item,