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

backend/src/modules/aia/services/conversationService.ts

@@ -17,6 +17,7 @@ import { streamChat, createStreamingService } from '../../../common/streaming/in
 import type { OpenAIMessage, StreamOptions } from '../../../common/streaming/index.js';
 import * as agentService from './agentService.js';
 import * as attachmentService from './attachmentService.js';
+import { activityService } from '../../../common/services/activity.service.js';
 import type { 
   Conversation, 
   Message, 
@@ -360,6 +361,27 @@ export async function sendMessageStream(
           tokens: aiMessage.tokens,
           hasThinking: !!thinkingContent,
         });
+
+        // 9. 埋点：记录智能体使用
+        try {
+          const user = await prisma.user.findUnique({
+            where: { id: userId },
+            include: { tenants: true }
+          });
+          if (user) {
+            const agent = await agentService.getAgentById(conversation.agentId);
+            activityService.log(
+              user.tenant_id,
+              user.tenants?.name || null,
+              userId,
+              user.name,
+              'AIA',
+              agent?.name || conversation.agentId,
+              'USE',
+              `tokens: ${aiMessage.tokens}`
+            );
+          }
+        } catch (e) { /* 埋点失败不影响主业务 */ }
       },
       onError: (error) => {
         logger.error('[AIA:ConversationService] 流式生成失败', {