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/agent/executor.go

@@ -107,6 +107,14 @@ func (e *Executor) ExecuteRunTask(ctx context.Context, taskID, recordID uint) er
 			return compressErr
 		}
 		finalPath = compressedPath
+	} else if strings.EqualFold(spec.Compression, "zstd") && !strings.HasSuffix(strings.ToLower(finalPath), ".zst") {
+		e.appendLog(ctx, recordID, "[agent] 开始压缩备份文件（zstd）\n")
+		compressedPath, compressErr := compress.ZstdFile(finalPath)
+		if compressErr != nil {
+			e.reportRecordFailure(ctx, recordID, fmt.Sprintf("压缩失败: %v", compressErr))
+			return compressErr
+		}
+		finalPath = compressedPath
 	}
 	info, err := os.Stat(finalPath)
 	if err != nil {
@@ -414,6 +422,15 @@ func (e *Executor) ExecuteRestore(ctx context.Context, restoreRecordID uint) err
 		}
 		preparedPath = decompressed
 	}
+	if strings.HasSuffix(strings.ToLower(preparedPath), ".zst") {
+		e.appendRestoreLog(ctx, restoreRecordID, "[agent] 解压 zstd 压缩\n")
+		decompressed, err := compress.UnzstdFile(preparedPath)
+		if err != nil {
+			e.reportRestoreFailure(ctx, restoreRecordID, fmt.Sprintf("解压失败: %v", err))
+			return err
+		}
+		preparedPath = decompressed
+	}
 
 	// 4) 运行 runner.Restore
 	taskSpec := buildRestoreBackupTaskSpec(spec, time.Now().UTC(), tmpDir)