feat(admin): Implement System Knowledge Base management module

Features:

- Backend: SystemKbService with full CRUD (knowledge bases + documents)

- Backend: 8 RESTful API endpoints (list/detail/create/update/delete/upload/download)

- Backend: OSS storage integration (system/knowledge-bases/{kbId}/{docId})

- Backend: RAG engine integration (document parsing, chunking, vectorization)

- Frontend: SystemKbListPage with card-based layout

- Frontend: SystemKbDetailPage with document management table

- Frontend: Master-Detail UX pattern for better user experience

- Document upload (single/batch), download (preserving original filename), delete

Technical:

- Database migration for system_knowledge_bases and system_kb_documents tables

- OSSAdapter.getSignedUrl with Content-Disposition for original filename

- Reuse RAG engine from common/rag for document processing

Tested: Local environment verified, all features working

frontend-v2/src/framework/layout/AdminLayout.tsx

@@ -12,6 +12,7 @@ import {
   MenuFoldOutlined,
   MenuUnfoldOutlined,
   BellOutlined,
+  BookOutlined,
 } from '@ant-design/icons'
 import type { MenuProps } from 'antd'
 import { useAuth } from '../auth'
@@ -83,6 +84,11 @@ const AdminLayout = () => {
       icon: <CodeOutlined />,
       label: 'Prompt管理',
     },
+    {
+      key: '/admin/system-kb',
+      icon: <BookOutlined />,
+      label: '系统知识库',
+    },
     {
       key: '/admin/tenants',
       icon: <TeamOutlined />,