Add ability to use service accounts in MongoDB

Currently, we are using MongoDB as root (not passing credentials when creating a `MongoClient` instance). This might pose some security issues, as the account used by the service should have restricted access (i.e. read only, read/write, but not delete).

We need to add credentials to configuration file, update deployments when needed.

首先定位 MongoClient 的创建位置以及部署所使用的配置；任务描述未指定任何文件或测试。追踪 MongoDB 设置的提供方式，然后确定应如何表示受限的服务账户角色，并验证部署能够在不授予不必要访问权限的情况下完成身份验证。

由索引模型根据 Issue 内容生成。