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/framework/layout/TopNavigation.tsx

@@ -102,7 +102,11 @@ const TopNavigation = () => {
         className="flex items-center gap-3 cursor-pointer"
         onClick={() => navigate('/')}
       >
-        <div className="text-2xl">🏥</div>
+        <img 
+          src="/logo.jpg" 
+          alt="AI临床研究平台" 
+          className="h-[52px] w-auto"
+        />
         <span className="text-xl font-bold text-blue-600">AI临床研究平台</span>
       </div>