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/config/app.php

@@ -124,5 +124,6 @@ return [
     ],
 
     'frontend_app_url' => env('FRONTEND_APP_URL', 'http://localhost:5174'),
+    'frontend_portal_url' => env('FRONTEND_PORTAL_URL', 'http://localhost:5175'),
 
 ];