Summary
Refactor and unify the way joinTo() and joinThrough() work across Quantum's database adapters (SleekDB, Idiorm, etc.) by introducing a declarative relations() system inside models. This will improve clarity, consistency, and maintainability of model relationships.

Objectives

• Replace legacy $foreignKeys array with a relations() method in each model.
• Introduce/refine joinTo() and joinThrough() methods to resolve relationships based on relations().
• Support chainable joins (e.g., User → UserEvent → Event) with clear control.
• Ensure join behavior works consistently across all supported adapters.

Design Notes

relations() format
Each model defines its foreign key relations like:

public function relations(): array
{
    return [
        RelatedModel::class => [
            'local_key' => 'id',
            'foreign_key' => 'user_id'
        ]
    ];
}

All relevant unit tests need to be updated.