fix(snapshot): API 响应补上 progressMsg 字段

toSnapshotResponse 手动映射字段时遗漏了 progressMsg，
导致前端轮询拿不到 MB 进度消息。

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

backend/services/snapshot-service/src/api/dto/snapshot.response.ts

@@ -5,6 +5,7 @@ export class SnapshotDetailResponse {
   @ApiProperty() target: string;
   @ApiProperty() status: string;
   @ApiProperty() progress: number;
+  @ApiProperty({ required: false }) progressMsg: string | null;
   @ApiProperty() fileSize: string;
   @ApiProperty() fileName: string | null;
   @ApiProperty() error: string | null;
@@ -52,6 +53,7 @@ export function toSnapshotResponse(task: any): SnapshotTaskResponse {
       target: d.target,
       status: d.status,
       progress: d.progress,
+      progressMsg: d.progressMsg ?? null,
       fileSize: d.fileSize.toString(),
       fileName: d.fileName,
       error: d.error,