refactor(posts): drop hardcoded English validation messages

Removes the messages() override on UpdatePostRequest. The 4 messages it
defined were hardcoded English strings that bypassed Laravel's built-in
validation translations. With the override gone, errors flow through
lang/{locale}/validation.php which is already translated for en/pt-BR/es.
Trade-off: default messages reference the field path (e.g.
'platforms.0.content_type'). Acceptable here because the editor renders
errors inline next to each field rather than as a flat list.

app/Http/Requests/App/Post/UpdatePostRequest.php

@@ -72,16 +72,6 @@ public function rules(): array
         ];
     }
 
-    public function messages(): array
-    {
-        return [
-            'status.required' => 'The post status is required.',
-            'status.in' => 'Invalid post status.',
-            'platforms.*.content_type.in' => 'Invalid content type.',
-            'scheduled_at.after' => 'The scheduled date must be in the future.',
-        ];
-    }
-
     public function withValidator(Validator $validator): void
     {
         $validator->after(function ($validator): void {