Providers

Named validation rules live in ValidationServiceProvider

Add a new named validation rule as a Validator::extend() (or extendImplicit()) closure in app/Providers/ValidationServiceProvider.php, then reference it by its string name in $rules.

app/Rules is reserved for the encrypted-custom-field rule objects — do not add general-purpose rules there.

Register observers in AppServiceProvider

Wire an observer with Model::observe(ModelObserver::class) in AppServiceProvider::boot(). Do not use the #[ObservedBy] attribute on the model.