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/pkg/compress/zstd.go

@@ -0,0 +1,65 @@
+package compress
+
+import (
+	"fmt"
+	"io"
+	"os"
+	"strings"
+
+	"github.com/klauspost/compress/zstd"
+)
+
+// ZstdFile 将文件压缩为 .zst（zstd），返回压缩产物路径。
+// 相比 gzip，zstd 在相近 CPU 开销下提供更高压缩率与显著更快的解压速度。
+func ZstdFile(sourcePath string) (string, error) {
+	source, err := os.Open(sourcePath)
+	if err != nil {
+		return "", fmt.Errorf("open source file: %w", err)
+	}
+	defer source.Close()
+	targetPath := sourcePath + ".zst"
+	target, err := os.Create(targetPath)
+	if err != nil {
+		return "", fmt.Errorf("create zstd file: %w", err)
+	}
+	defer target.Close()
+	writer, err := zstd.NewWriter(target)
+	if err != nil {
+		return "", fmt.Errorf("create zstd writer: %w", err)
+	}
+	if _, err := io.Copy(writer, source); err != nil {
+		_ = writer.Close()
+		return "", fmt.Errorf("zstd source file: %w", err)
+	}
+	if err := writer.Close(); err != nil {
+		return "", fmt.Errorf("close zstd writer: %w", err)
+	}
+	return targetPath, nil
+}
+
+// UnzstdFile 解压 .zst 文件，返回解压产物路径。
+func UnzstdFile(sourcePath string) (string, error) {
+	source, err := os.Open(sourcePath)
+	if err != nil {
+		return "", fmt.Errorf("open zstd file: %w", err)
+	}
+	defer source.Close()
+	reader, err := zstd.NewReader(source)
+	if err != nil {
+		return "", fmt.Errorf("create zstd reader: %w", err)
+	}
+	defer reader.Close()
+	targetPath := strings.TrimSuffix(sourcePath, ".zst")
+	if targetPath == sourcePath {
+		targetPath += ".out"
+	}
+	target, err := os.Create(targetPath)
+	if err != nil {
+		return "", fmt.Errorf("create target file: %w", err)
+	}
+	defer target.Close()
+	if _, err := io.Copy(target, reader); err != nil {
+		return "", fmt.Errorf("unzstd file: %w", err)
+	}
+	return targetPath, nil
+}