feat(backup): 新增 zstd 压缩选项 (#89)

备份压缩在 gzip 之外新增 zstd（更高压缩率、更快解压）。pkg/compress 新增 ZstdFile/UnzstdFile，Master 与 Agent 压缩/解压按后缀分流，任务校验与前端下拉同步；往返单测覆盖。

server/internal/service/execution_helpers.go

@@ -106,6 +106,16 @@ func prepareBackupArtifact(cipher *codec.ConfigCipher, artifactPath string, logg
 		}
 		current = decompressed
 	}
+	if strings.HasSuffix(strings.ToLower(current), ".zst") {
+		if logger != nil {
+			logger.Infof("检测到 zstd 压缩，开始解压")
+		}
+		decompressed, err := compress.UnzstdFile(current)
+		if err != nil {
+			return "", err
+		}
+		current = decompressed
+	}
 	return current, nil
 }