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/index.ts

@@ -108,12 +108,14 @@ 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';
+import { systemKbRoutes } from './modules/admin/system-kb/index.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' });
 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');
+await fastify.register(systemKbRoutes, { prefix: '/api/v1/admin/system-kb' });
+logger.info('✅ 运营管理路由已注册: /api/admin/tenants, /api/admin/modules, /api/admin/users, /api/admin/stats, /api/v1/admin/system-kb');
 
 // ============================================
 // 【临时】平台基础设施测试API