feat(nex-be): 添加快照响应 DTO 类

- 新增 SnapshotsResponseDTO 类用于快照信息响应
- 包含总记录数、分页信息和快照列表
- 定义了快照项的嵌套类，包含快照详细信息

nex-be/src/main/java/com/unisinsight/project/entity/dto/SnapshotsResponseDTO.java

@@ -0,0 +1,24 @@
+package com.unisinsight.project.entity.dto;
+
+import lombok.Data;
+import java.util.List;
+
+@Data
+public class SnapshotsResponseDTO {
+    private List<SnapshotItem> items;
+    private Integer total;
+    private Integer page;
+    private Integer page_size;
+    private String message;
+
+    @Data
+    public static class SnapshotItem {
+        private String name;
+        private String description;
+        private String snapshot_type;
+        private Long creation_time;
+        private String state;
+        private String vm_name;
+        private String creation_time_formatted;
+    }
+}