feat(api): registration auth, account creation, check-email & email notifications

- Add POST /public/check-email endpoint with rate limiting (10/min)
- Create user accounts during volunteer registration (new or returning)
- Returning volunteers authenticate with existing password
- Add password validation to VolunteerRegistrationRequest
- Normalize emails to lowercase throughout registration flow
- Handle race condition on duplicate accounts gracefully
- Create RegistrationConfirmationMail, RegistrationApprovedMail, RegistrationRejectedMail
- Wire approval/rejection emails into PersonController
- Add POST persons/{person}/reject endpoint
- Trigger TagSyncService on registration and approval
- Add CheckEmailTest, PersonApprovalEmailTest, extend VolunteerRegistrationTest

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

api/app/Mail/RegistrationRejectedMail.php

@@ -0,0 +1,45 @@
+<?php
+
+declare(strict_types=1);
+
+namespace App\Mail;
+
+use App\Models\Event;
+use App\Models\Person;
+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 RegistrationRejectedMail extends Mailable implements ShouldQueue
+{
+    use Queueable;
+    use SerializesModels;
+
+    public function __construct(
+        public readonly Person $person,
+        public readonly Event $event,
+        public readonly ?string $reason = null,
+    ) {}
+
+    public function envelope(): Envelope
+    {
+        return new Envelope(
+            subject: "Update over je aanmelding — {$this->event->name}",
+        );
+    }
+
+    public function content(): Content
+    {
+        return new Content(
+            markdown: 'emails.registration-rejected',
+            with: [
+                'personName' => $this->person->first_name,
+                'eventName' => $this->event->name,
+                'reason' => $this->reason,
+            ],
+        );
+    }
+}