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/tests/frontdoor/public_support.rs

@@ -3871,7 +3871,13 @@ async fn gateway_handles_wallet_balance_locally_without_proxying_upstream() {
 
 #[tokio::test]
 async fn gateway_handles_wallet_today_cost_locally_without_proxying_upstream() {
-    let now = Utc::now();
+    let now = chrono::DateTime::<chrono::Utc>::from_naive_utc_and_offset(
+        Utc::now()
+            .date_naive()
+            .and_hms_opt(12, 0, 0)
+            .expect("midday should be valid"),
+        chrono::Utc,
+    );
     let user = sample_auth_user(now);
     let access_token = build_test_auth_token(
         "access",