fix: 获取备份路径时自动创建目录

修复上传 MySQL 等备份文件时，因备份目录（如 /backup/mysql/）不存在导致
报错"文件不存在"的问题。将创建目录逻辑收敛到 GetDefaultPath，所有调用方均受益。

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

internal/data/backup.go

@@ -260,9 +260,11 @@ func (r *backupRepo) Restore(ctx context.Context, typ biz.BackupType, backup, ta
 func (r *backupRepo) GetDefaultPath(typ biz.BackupType) string {
 	backupPath, err := r.setting.Get(biz.SettingKeyBackupPath)
 	if err != nil {
-		return filepath.Join(app.Root, "backup", string(typ))
+		backupPath = filepath.Join(app.Root, "backup")
 	}
-	return filepath.Join(backupPath, string(typ))
+	path := filepath.Join(backupPath, string(typ))
+	_ = os.MkdirAll(path, 0755)
+	return path
 }
 
 // CutoffLog 切割日志