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/config/config.go

@@ -85,7 +85,9 @@ func (c *Config) Validate() error {
 		if p.ComposeFile == "" {
 			p.ComposeFile = "docker-compose.yml"
 		}
-		if p.Retention.Count <= 0 {
+		// Apply the default count only when no retention policy is set at all.
+		// This lets a user configure days-only retention (count omitted).
+		if p.Retention.Count <= 0 && p.Retention.Days <= 0 {
 			p.Retention.Count = DefaultRetentionCount
 		}
 	}

internal/config/config_test.go

@@ -45,6 +45,34 @@ projects:
 	}
 }
 
+func TestLoadDaysOnlyRetention(t *testing.T) {
+	yaml := `
+global:
+  backup_dir: /tmp/backups
+projects:
+  - name: testapp
+    path: /opt/testapp
+    retention:
+      days: 30
+`
+	dir := t.TempDir()
+	path := filepath.Join(dir, "config.yaml")
+	if err := os.WriteFile(path, []byte(yaml), 0644); err != nil {
+		t.Fatal(err)
+	}
+	cfg, err := Load(path)
+	if err != nil {
+		t.Fatalf("Load: %v", err)
+	}
+	p := cfg.Projects[0]
+	if p.Retention.Count != 0 {
+		t.Errorf("count should stay 0 for days-only retention, got %d", p.Retention.Count)
+	}
+	if p.Retention.Days != 30 {
+		t.Errorf("days = %d", p.Retention.Days)
+	}
+}
+
 func TestLoadMissingBackupDir(t *testing.T) {
 	yaml := `
 projects: