fix: 修复proxy_service和account_service中不当的顶层import

- proxy_service.py: 将requests和WhitelistManager改为函数内延迟import，
  避免启动时加载不需要的依赖；移除未使用的get_exit_ip_via_proxy导入
- account_service.py: 移除未使用的func、joinedload、AuditLog、
  user_has_permission顶层导入

web/frontend/src/pages/AssignmentsPage.tsx

@@ -30,7 +30,7 @@ export default function AssignmentsPage() {
     try {
       const data = await accountApi.assignmentsSummary();
       setSupportUsers(data.support_users);
-      return data.support_users as SupportUser[];
+      return data.support_users;
     } catch (e: unknown) {
       message.error(getErrorMessage(e));
       return [];
@@ -54,7 +54,7 @@ export default function AssignmentsPage() {
       const users = await loadSummary();
       if (users.length > 0) {
         const savedId = localStorage.getItem(STORAGE_KEY_SELECTED_USER);
-        const savedUser = savedId ? users.find((u: SupportUserItemItem) => u.id === Number(savedId)) : null;
+        const savedUser = savedId ? users.find((u: SupportUserItem) => u.id === Number(savedId)) : null;
         setSelectedUser(savedUser || users[0]);
       }
     };
@@ -394,4 +394,4 @@ export default function AssignmentsPage() {
       </Row>
     </div>
   );
-}
+}