feat: Observable Model Properties (#[Watches] attribute system)

[techmahedy]

This PR introduces Observable Model Properties — a property-level reactive observation system for Doppar models, implemented via the #[Watches] attribute.

When a watched model property is dirty at save() or update() time, the framework automatically resolves the registered watcher from the DI container and calls its handle($old, $new, $model) method. An optional when condition (either a model method or a class implementing WatchConditionInterface) controls whether the listener fires.

A make:watcher console command scaffolds new watcher listener classes into app/Watchers/.

Example

#[Watches(EmployeeStatusWatcher::class, when: 'isFraudRisk')]
protected bool $status;

public function isFraudRisk(): bool
{
      return true;
}