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/index.ts

@@ -82,6 +82,13 @@ console.log('✅ 文件上传插件已配置: 最大文件大小 10MB');
 await registerHealthRoutes(fastify);
 logger.info('✅ 健康检查路由已注册');
 
+// ============================================
+// 【公开API】无需认证的公共接口
+// ============================================
+import { getTenantLoginConfig } from './modules/admin/controllers/tenantController.js';
+fastify.get('/api/v1/public/tenant-config/:tenantCode', getTenantLoginConfig);
+logger.info('✅ 公开API已注册: /api/v1/public/tenant-config/:tenantCode');
+
 // ============================================
 // 【平台基础设施】认证模块
 // ============================================
@@ -100,10 +107,13 @@ logger.info('✅ Prompt管理路由已注册: /api/admin/prompts');
 // ============================================
 import { tenantRoutes, moduleRoutes } from './modules/admin/routes/tenantRoutes.js';
 import { userRoutes } from './modules/admin/routes/userRoutes.js';
+import { statsRoutes, userOverviewRoute } from './modules/admin/routes/statsRoutes.js';
 await fastify.register(tenantRoutes, { prefix: '/api/admin/tenants' });
 await fastify.register(moduleRoutes, { prefix: '/api/admin/modules' });
 await fastify.register(userRoutes, { prefix: '/api/admin/users' });
-logger.info('✅ 运营管理路由已注册: /api/admin/tenants, /api/admin/modules, /api/admin/users');
+await fastify.register(statsRoutes, { prefix: '/api/admin/stats' });
+await fastify.register(userOverviewRoute, { prefix: '/api/admin/users' });
+logger.info('✅ 运营管理路由已注册: /api/admin/tenants, /api/admin/modules, /api/admin/users, /api/admin/stats');
 
 // ============================================
 // 【临时】平台基础设施测试API