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

config.example.yaml

@@ -1,7 +1,10 @@
 # docker-compose-backup example configuration
 global:
   backup_dir: /opt/docker-compose-backup/backups
-  # temp_dir: /tmp/docker-compose-backup  # defaults to OS temp dir
+  # Staging area for the pre-compression copy. Defaults to the OS temp dir.
+  # Under the bundled systemd unit, set this inside the install dir so it stays
+  # within the sandbox's writable path (the unit no longer uses PrivateTmp).
+  temp_dir: /opt/docker-compose-backup/tmp
 
 projects:
   - name: myapp