feat(data-management): 添加知识项扩展元数据支持

- 在 KnowledgeItemApplicationService 中实现元数据字段的更新逻辑
- 为 CreateKnowledgeItemRequest 添加 metadata 字段定义
- 为 UpdateKnowledgeItemRequest 添加 metadata 字段定义
- 支持知识项创建和更新时的扩展元数据存储

backend/services/data-management-service/src/main/java/com/datamate/datamanagement/application/KnowledgeItemApplicationService.java

@@ -178,6 +178,9 @@ public class KnowledgeItemApplicationService {
         if (request.getContentType() != null) {
             knowledgeItem.setContentType(request.getContentType());
         }
+        if (request.getMetadata() != null) {
+            knowledgeItem.setMetadata(request.getMetadata());
+        }
 
         knowledgeItemRepository.updateById(knowledgeItem);
         return knowledgeItem;

backend/services/data-management-service/src/main/java/com/datamate/datamanagement/interfaces/dto/CreateKnowledgeItemRequest.java

@@ -34,4 +34,8 @@ public class CreateKnowledgeItemRequest {
      * 来源文件ID（用于标注同步等场景）
      */
     private String sourceFileId;
+    /**
+     * 扩展元数据
+     */
+    private String metadata;
 }

backend/services/data-management-service/src/main/java/com/datamate/datamanagement/interfaces/dto/UpdateKnowledgeItemRequest.java

@@ -18,4 +18,8 @@ public class UpdateKnowledgeItemRequest {
      * 内容类型
      */
     private KnowledgeContentType contentType;
+    /**
+     * 扩展元数据
+     */
+    private String metadata;
 }