fix: 修复备份工具的多处问题

- go.mod: 将非法版本号 go 1.26.4 改为 go 1.26
- engine: 压缩失败时删除残留的半截 tar.gz 文件
- config: 支持仅按天数保留(count 和 days 均未设时才用默认 count=7)
- engine: docker compose stop 失败时也尝试重启,避免容器停摆
- list: 仅显示真正的备份归档文件(新增导出 IsBackupArchive)
- systemd: 移除 PrivateTmp,暂存目录改到安装目录内,避免私有 /tmp 被大目录撑爆

internal/backup/retention.go

@@ -87,5 +87,10 @@ func ApplyRetention(backupDir, projectName string, policy RetentionPolicy) (int,
 }
 
 func isBackupArchive(projectName, name string) bool {
+	return IsBackupArchive(projectName, name)
+}
+
+// IsBackupArchive reports whether name is a backup archive for the project.
+func IsBackupArchive(projectName, name string) bool {
 	return strings.HasPrefix(name, projectName+"-") && strings.HasSuffix(name, ".tar.gz")
 }