feat: delete subscribers from page subscriber list

Adds DELETE route, form request authorization, admin UI with confirm, Dutch strings, and feature tests.

Made-with: Cursor

app/Http/Requests/Admin/DestroySubscriberRequest.php

@@ -0,0 +1,36 @@
+<?php
+
+declare(strict_types=1);
+
+namespace App\Http\Requests\Admin;
+
+use App\Models\PreregistrationPage;
+use App\Models\Subscriber;
+use Illuminate\Contracts\Validation\ValidationRule;
+use Illuminate\Foundation\Http\FormRequest;
+
+class DestroySubscriberRequest extends FormRequest
+{
+    public function authorize(): bool
+    {
+        $page = $this->route('page');
+        $subscriber = $this->route('subscriber');
+        if (! $page instanceof PreregistrationPage || ! $subscriber instanceof Subscriber) {
+            return false;
+        }
+
+        if ($subscriber->preregistration_page_id !== $page->id) {
+            return false;
+        }
+
+        return $this->user()?->can('update', $page) ?? false;
+    }
+
+    /**
+     * @return array<string, array<int, ValidationRule|string>>
+     */
+    public function rules(): array
+    {
+        return [];
+    }
+}