Parent issue: #164

The API endpoints to return the list of categories:

http://127.0.0.1:3000/v1/category

returns the struct Category:

pub struct Category {
    pub category_id: i64,
    pub name: String,
    pub num_torrents: i64,
}

We should add a DTO for the category in the delivery layer.