feat(backup): 新增 MongoDB 备份与恢复支持 (#87)

通过 mongodump/mongorestore --archive 流式管线接入 MongoDB 数据源，与现有数据库运行器架构一致；注册到 Master 与 Agent，含任务校验、默认端口与前端表单/恢复确认。5 个单测覆盖参数构造、全库、空产物与缺工具分支。

server/internal/service/backup_task_service.go

@@ -21,7 +21,7 @@ const backupTaskMaskedValue = "********"
 
 type BackupTaskUpsertInput struct {
 	Name             string   `json:"name" binding:"required,min=1,max=100"`
-	Type             string   `json:"type" binding:"required,oneof=file mysql sqlite postgresql pgsql saphana"`
+	Type             string   `json:"type" binding:"required,oneof=file mysql sqlite postgresql pgsql saphana mongodb"`
 	Enabled          bool     `json:"enabled"`
 	CronExpr         string   `json:"cronExpr" binding:"max=64"`
 	SourcePath       string   `json:"sourcePath" binding:"max=500"`
@@ -578,7 +578,7 @@ func validateTaskTypeSpecificFields(input BackupTaskUpsertInput, passwordRequire
 		if !hasSourcePaths {
 			return apperror.BadRequest("BACKUP_TASK_INVALID", "文件备份必须填写源路径", nil)
 		}
-	case "mysql", "postgresql", "saphana":
+	case "mysql", "postgresql", "saphana", "mongodb":
 		if strings.TrimSpace(input.DBHost) == "" {
 			return apperror.BadRequest("BACKUP_TASK_INVALID", "数据库主机不能为空", nil)
 		}