feat: system knowledge base management with admin/user role separation

- Auto-create system KBs from data/knowledge_base/ subdirectories on startup
- Upgrade existing user KBs to system KBs when matching directory found
- Admin can upload to system KBs (saved to knowledge_base dir)
- Regular users see only their own KBs on knowledge page
- Fix API permission checks for system KBs (list, detail, documents, update, delete)
- Add is_superuser to UserResponse and frontend User type
- Fix sidebar session menu visibility for long titles

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

backend/src/services/file_watcher_service.py

@@ -132,6 +132,10 @@ class FileWatcherService:
             
             print(f"文件监控服务已启动，监控目录: {self.knowledge_base_dir}")
             
+            # 确保子目录对应的系统知识库存在
+            print("确保系统知识库与目录同步...")
+            kb_service.ensure_system_knowledge_bases()
+
             # 执行初始扫描
             print("执行初始知识库扫描...")
             scan_result = kb_service.scan_directory()