Document how to use UUID with Auth better

If you generate Auth in your API app, you'll get an UserToken class that assumes you have a User model that uses Int64 for your primary key. If you decide to use UUID, you'll need to make a few changes.

• Update migration to set primary key to UUID
• Add default_columns macro to base_model with UUID primary key
• Change return type on decode_user_id method in UserToken class to UUID
• Change the return value from payload["user_id"].to_s.to_i64 to UUID.new(payload["user_id"].to_s)

I think this is all you need to do, but we should have a section somewhere on the site that documents this.

Start by locating the website's existing Auth or API-app documentation and the UserToken section. Review how decode_user_id and UUID-based models are currently described, then add a dedicated section covering the migration, default_columns, and UUID return-value changes. Done means a newcomer can follow the section to configure Auth with UUID primary keys.