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

backend/src/modules/admin/system-kb/systemKbRoutes.ts

@@ -0,0 +1,75 @@
+/**
+ * 系统知识库路由
+ * 
+ * 路由前缀：/api/v1/admin/system-kb
+ */
+
+import { FastifyInstance } from 'fastify';
+import {
+  listKnowledgeBases,
+  getKnowledgeBase,
+  createKnowledgeBase,
+  updateKnowledgeBase,
+  deleteKnowledgeBase,
+  listDocuments,
+  uploadDocument,
+  deleteDocument,
+  downloadDocument,
+} from './systemKbController.js';
+import { authenticate, requireRoles } from '../../../common/auth/auth.middleware.js';
+
+export async function systemKbRoutes(fastify: FastifyInstance) {
+  // 所有路由都需要认证 + SUPER_ADMIN 或 ADMIN 角色
+  const preHandler = [authenticate, requireRoles('SUPER_ADMIN', 'ADMIN')];
+
+  // ==================== 知识库 CRUD ====================
+
+  // 获取知识库列表
+  fastify.get('/', {
+    preHandler,
+  }, listKnowledgeBases as any);
+
+  // 创建知识库
+  fastify.post('/', {
+    preHandler,
+  }, createKnowledgeBase as any);
+
+  // 获取知识库详情
+  fastify.get('/:id', {
+    preHandler,
+  }, getKnowledgeBase as any);
+
+  // 更新知识库
+  fastify.patch('/:id', {
+    preHandler,
+  }, updateKnowledgeBase as any);
+
+  // 删除知识库
+  fastify.delete('/:id', {
+    preHandler,
+  }, deleteKnowledgeBase as any);
+
+  // ==================== 文档管理 ====================
+
+  // 获取文档列表
+  fastify.get('/:id/documents', {
+    preHandler,
+  }, listDocuments as any);
+
+  // 上传文档
+  fastify.post('/:id/documents', {
+    preHandler,
+  }, uploadDocument as any);
+
+  // 删除文档
+  fastify.delete('/:id/documents/:docId', {
+    preHandler,
+  }, deleteDocument as any);
+
+  // 下载文档（获取签名 URL）
+  fastify.get('/:id/documents/:docId/download', {
+    preHandler,
+  }, downloadDocument as any);
+}
+
+export default systemKbRoutes;