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/App.tsx

@@ -21,6 +21,9 @@ import { MODULES } from './framework/modules/moduleRegistry'
 import UserListPage from './modules/admin/pages/UserListPage'
 import UserFormPage from './modules/admin/pages/UserFormPage'
 import UserDetailPage from './modules/admin/pages/UserDetailPage'
+// 系统知识库管理
+import SystemKbListPage from './modules/admin/pages/SystemKbListPage'
+import SystemKbDetailPage from './modules/admin/pages/SystemKbDetailPage'
 // 个人中心页面
 import ProfilePage from './pages/user/ProfilePage'
 
@@ -109,6 +112,9 @@ function App() {
                   <Route path="users/create" element={<UserFormPage mode="create" />} />
                   <Route path="users/:id" element={<UserDetailPage />} />
                   <Route path="users/:id/edit" element={<UserFormPage mode="edit" />} />
+                  {/* 系统知识库 */}
+                  <Route path="system-kb" element={<SystemKbListPage />} />
+                  <Route path="system-kb/:id" element={<SystemKbDetailPage />} />
                   {/* 系统配置 */}
                   <Route path="system" element={<div className="text-center py-20">🚧 系统配置页面开发中...</div>} />
                 </Route>