feat(admin): Implement operational monitoring MVP and login optimization

Summary:
- Add SimpleLog table for activity tracking (admin_schema)
- Implement ActivityService with fire-and-forget pattern
- Add stats API endpoints (overview/live-feed/user-overview/cleanup)
- Complete activity logging for 7 modules (SYSTEM/AIA/PKB/ASL/DC/RVW/IIT)
- Update Admin Dashboard with DAU/DAT metrics and live feed
- Fix user module permission display logic
- Fix login redirect to /ai-qa instead of homepage
- Replace top navigation LOGO with brand image
- Fix PKB workspace layout CSS conflict (rename to .pa-chat-container)

New files:
- backend/src/common/services/activity.service.ts
- backend/src/modules/admin/controllers/statsController.ts
- backend/src/modules/admin/routes/statsRoutes.ts
- frontend-v2/src/modules/admin/api/statsApi.ts
- docs/03-.../04-operational-monitoring-mvp-plan.md
- docs/03-.../04-operational-monitoring-mvp-implementation.md

Tested: All features verified locally

frontend-v2/src/modules/admin/index.tsx

@@ -2,6 +2,7 @@
  * ADMIN（运营管理端）模块入口
  * 
  * 功能：
+ * - 运营监控看板
  * - 用户管理
  * - 租户管理（已有）
  * - Prompt管理（已有）
@@ -12,11 +13,15 @@ import { Routes, Route, Navigate } from 'react-router-dom';
 import UserListPage from './pages/UserListPage';
 import UserFormPage from './pages/UserFormPage';
 import UserDetailPage from './pages/UserDetailPage';
+import StatsDashboardPage from './pages/StatsDashboardPage';
 
 const AdminModule: React.FC = () => {
   return (
     <Routes>
-      <Route path="/" element={<Navigate to="users" replace />} />
+      <Route path="/" element={<Navigate to="stats" replace />} />
+      
+      {/* 运营监控看板 */}
+      <Route path="stats" element={<StatsDashboardPage />} />
       
       {/* 用户管理 */}
       <Route path="users" element={<UserListPage />} />