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/prisma/schema.prisma

@@ -970,6 +970,34 @@ model admin_operation_logs {
   @@schema("admin_schema")
 }
 
+/// 运营日志表 (MVP V3.1)
+/// 用于记录用户行为，支持 DAU/DAT 统计和用户360画像
+model simple_logs {
+  id         String   @id @default(dbgenerated("gen_random_uuid()")) @db.Uuid
+  created_at DateTime @default(now())
+
+  // 租户和用户信息
+  tenant_id   String  @db.VarChar(50)
+  tenant_name String? @db.VarChar(100) // 冗余字段，避免JOIN
+  user_id     String  @db.Uuid
+  user_name   String? @db.VarChar(50)
+
+  // 行为记录
+  module  String @db.VarChar(20) // AIA, PKB, ASL, DC, RVW, IIT, SSA, ST, SYSTEM
+  feature String @db.VarChar(50) // 细分功能
+  action  String @db.VarChar(20) // LOGIN, USE, EXPORT, CREATE, DELETE, ERROR
+
+  // 详情信息
+  info String? @db.Text // JSON或文本详情
+
+  @@index([created_at], map: "idx_simple_logs_created_at")
+  @@index([tenant_id], map: "idx_simple_logs_tenant_id")
+  @@index([user_id], map: "idx_simple_logs_user_id")
+  @@index([module, feature], map: "idx_simple_logs_module_feature")
+  @@index([action], map: "idx_simple_logs_action")
+  @@schema("admin_schema")
+}
+
 model departments {
   id                String        @id
   tenant_id         String