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/Requests/Api/V1/CheckEmailRequest.php

@@ -0,0 +1,23 @@
+<?php
+
+declare(strict_types=1);
+
+namespace App\Http\Requests\Api\V1;
+
+use Illuminate\Foundation\Http\FormRequest;
+
+final class CheckEmailRequest extends FormRequest
+{
+    public function authorize(): bool
+    {
+        return true;
+    }
+
+    /** @return array<string, mixed> */
+    public function rules(): array
+    {
+        return [
+            'email' => ['required', 'email'],
+        ];
+    }
+}

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

@@ -30,7 +30,9 @@ final class VolunteerRegistrationRequest extends FormRequest
     /** @return array<string, mixed> */
     public function rules(): array
     {
-        return [
+        $user = auth('sanctum')->user();
+
+        $rules = [
             'first_name' => ['required_without:_authenticated', 'string', 'max:255'],
             'last_name' => ['required_without:_authenticated', 'string', 'max:255'],
             'email' => ['required_without:_authenticated', 'email', 'max:255'],
@@ -55,5 +57,13 @@ final class VolunteerRegistrationRequest extends FormRequest
 
             'field_values' => ['nullable', 'array'],
         ];
+
+        // Password required for unauthenticated registrations
+        if ($user === null) {
+            $rules['password'] = ['required', 'string', 'min:8'];
+            $rules['password_confirmation'] = ['nullable', 'same:password'];
+        }
+
+        return $rules;
     }
 }