feat: fase 2 backend — crowd types, persons, sections, shifts, invite flow

- Crowd Types + Persons CRUD (73 tests)
- Festival Sections + Time Slots + Shifts CRUD met assign/claim flow (84 tests)
- Invite Flow + Member Management met InvitationService (109 tests)
- Schema v1.6 migraties volledig uitgevoerd
- DevSeeder bijgewerkt met crowd types voor testorganisatie

api/app/Http/Requests/Api/V1/AcceptInvitationRequest.php

@@ -0,0 +1,29 @@
+<?php
+
+declare(strict_types=1);
+
+namespace App\Http\Requests\Api\V1;
+
+use App\Models\User;
+use App\Models\UserInvitation;
+use Illuminate\Foundation\Http\FormRequest;
+
+final class AcceptInvitationRequest extends FormRequest
+{
+    public function authorize(): bool
+    {
+        return true;
+    }
+
+    /** @return array<string, mixed> */
+    public function rules(): array
+    {
+        $invitation = UserInvitation::where('token', $this->route('token'))->first();
+        $userExists = $invitation && User::where('email', $invitation->email)->exists();
+
+        return [
+            'name' => [$userExists ? 'nullable' : 'required', 'string', 'max:255'],
+            'password' => [$userExists ? 'nullable' : 'required', 'string', 'min:8', 'confirmed'],
+        ];
+    }
+}