Disallow mixins on Model

Mixins lead to mess in big projects, it is better to not allow them at all. Business logic should not reside in models with the caveat for hooks and validations.

The issue is about disallowing mixins on Model classes in JugglingDB. First, locate the Model base class definition and understand how mixins are currently applied. Then, identify all places where mixins are used or allowed, and plan how to enforce the restriction. The change likely involves modifying the core ORM code and could affect existing projects, so careful consideration of backward compatibility is needed.

Scritto dal modello di indicizzazione a partire dal testo della issue.