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/Mail/InvitationMail.php

@@ -0,0 +1,44 @@
+<?php
+
+declare(strict_types=1);
+
+namespace App\Mail;
+
+use App\Models\UserInvitation;
+use Illuminate\Bus\Queueable;
+use Illuminate\Contracts\Queue\ShouldQueue;
+use Illuminate\Mail\Mailable;
+use Illuminate\Mail\Mailables\Content;
+use Illuminate\Mail\Mailables\Envelope;
+use Illuminate\Queue\SerializesModels;
+
+final class InvitationMail extends Mailable implements ShouldQueue
+{
+    use Queueable;
+    use SerializesModels;
+
+    public function __construct(
+        public readonly UserInvitation $invitation,
+    ) {}
+
+    public function envelope(): Envelope
+    {
+        return new Envelope(
+            subject: "Je bent uitgenodigd voor {$this->invitation->organisation->name}",
+        );
+    }
+
+    public function content(): Content
+    {
+        return new Content(
+            markdown: 'emails.invitation',
+            with: [
+                'acceptUrl' => config('app.frontend_app_url') . '/invitations/' . $this->invitation->token . '/accept',
+                'organisationName' => $this->invitation->organisation->name,
+                'inviterName' => $this->invitation->invitedBy?->name ?? 'Een beheerder',
+                'role' => $this->invitation->role,
+                'expiresAt' => $this->invitation->expires_at,
+            ],
+        );
+    }
+}