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/Http/Controllers/Api/V1/CheckEmailController.php

@@ -0,0 +1,20 @@
+<?php
+
+declare(strict_types=1);
+
+namespace App\Http\Controllers\Api\V1;
+
+use App\Http\Controllers\Controller;
+use App\Http\Requests\Api\V1\CheckEmailRequest;
+use App\Models\User;
+use Illuminate\Http\JsonResponse;
+
+final class CheckEmailController extends Controller
+{
+    public function __invoke(CheckEmailRequest $request): JsonResponse
+    {
+        $exists = User::where('email', strtolower($request->validated('email')))->exists();
+
+        return response()->json(['exists' => $exists]);
+    }
+}