feat(knowledge): 添加知识条目导出功能和文件上传支持

- 在 KnowledgeItemApplicationService 中新增 exportKnowledgeItems 方法实现知识条目导出
- 添加 export 相关常量配置包括文件名格式、内容类型等
- 在 KnowledgeItemRepository 中新增 findAllBySetId 查询方法
- 在 KnowledgeItemController 中新增 export 接口端点
- 在 KnowledgeItemEditor 组件中添加文件上传功能支持 txt/md/markdown 格式
- 在 KnowledgeSetDetail 页面中添加导出按钮并集成导出 API
- 更新前端 API 文件添加 exportKnowledgeItemsUsingGet 方法
- 配置文件上传验证和自动填充标题内容逻辑

frontend/src/pages/KnowledgeManagement/Detail/KnowledgeSetDetail.tsx

@@ -11,7 +11,7 @@ import {
   Tag,
   Tooltip,
 } from "antd";
-import { DeleteOutlined, EditOutlined, EyeOutlined, PlusOutlined } from "@ant-design/icons";
+import { DeleteOutlined, DownloadOutlined, EditOutlined, EyeOutlined, PlusOutlined } from "@ant-design/icons";
 import { useNavigate, useParams } from "react-router";
 import DetailHeader from "@/components/DetailHeader";
 import { SearchControls } from "@/components/SearchControls";
@@ -19,6 +19,7 @@ import useFetchData from "@/hooks/useFetchData";
 import {
   deleteKnowledgeItemByIdUsingDelete,
   deleteKnowledgeSetByIdUsingDelete,
+  exportKnowledgeItemsUsingGet,
   queryKnowledgeItemsUsingGet,
   queryKnowledgeSetByIdUsingGet,
 } from "../knowledge-management.api";
@@ -100,6 +101,12 @@ const KnowledgeSetDetail = () => {
     fetchData();
   };
 
+  const handleExportItems = async () => {
+    if (!id) return;
+    await exportKnowledgeItemsUsingGet(id);
+    message.success("知识条目导出成功");
+  };
+
   const isReadableItem = (record: KnowledgeItemView) => {
     return (
       record.contentType === KnowledgeContentType.TEXT ||
@@ -286,6 +293,12 @@ const KnowledgeSetDetail = () => {
             onClick: () => setShowEdit(true),
             danger: false,
           },
+          {
+            key: "export",
+            label: "导出",
+            icon: <DownloadOutlined />,
+            onClick: handleExportItems,
+          },
           {
             key: "delete",
             label: "删除",