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/aia/protocol-agent/styles/protocol-agent.css

@@ -411,8 +411,8 @@
   overflow: hidden;
 }
 
-/* 聊天容器 */
-.chat-container {
+/* 聊天容器 - 使用 pa- 前缀避免与共享组件 ChatContainer 冲突 */
+.pa-chat-container {
   flex: 1;
   min-height: 0; /* 关键：允许 flex 子元素收缩 */
   overflow-y: auto;
@@ -1838,7 +1838,7 @@
 /* ============================================ */
 /* 滚动条美化 - 始终可见 */
 /* ============================================ */
-.chat-container,
+.pa-chat-container,
 .panel-body,
 .conversations-list,
 .document-scroll-area {
@@ -1846,14 +1846,14 @@
   scrollbar-color: #94A3B8 #F1F5F9;
 }
 
-.chat-container::-webkit-scrollbar,
+.pa-chat-container::-webkit-scrollbar,
 .panel-body::-webkit-scrollbar,
 .conversations-list::-webkit-scrollbar,
 .document-scroll-area::-webkit-scrollbar {
   width: 8px;
 }
 
-.chat-container::-webkit-scrollbar-track,
+.pa-chat-container::-webkit-scrollbar-track,
 .panel-body::-webkit-scrollbar-track,
 .conversations-list::-webkit-scrollbar-track,
 .document-scroll-area::-webkit-scrollbar-track {
@@ -1861,7 +1861,7 @@
   border-radius: 4px;
 }
 
-.chat-container::-webkit-scrollbar-thumb,
+.pa-chat-container::-webkit-scrollbar-thumb,
 .panel-body::-webkit-scrollbar-thumb,
 .conversations-list::-webkit-scrollbar-thumb,
 .document-scroll-area::-webkit-scrollbar-thumb {
@@ -1870,7 +1870,7 @@
   border: 2px solid #F1F5F9;
 }
 
-.chat-container::-webkit-scrollbar-thumb:hover,
+.pa-chat-container::-webkit-scrollbar-thumb:hover,
 .panel-body::-webkit-scrollbar-thumb:hover,
 .conversations-list::-webkit-scrollbar-thumb:hover,
 .document-scroll-area::-webkit-scrollbar-thumb:hover {