feat: implement Story 4.2 - New Participant Registration

Implement participant registration with the following features:

- POST handler for /exchange/<slug>/register
- Create Participant record with lowercased email
- Generate magic token and send confirmation email
- Redirect to success page after registration
- Rate limiting: 10 registrations per hour per IP
- Validation for exchange state (must be registration_open)
- Form validation for required fields (name, email)
- Email format validation
- Optional fields support (gift_ideas, reminder_enabled)

Also includes:
- Registration success page template
- 429 error handling template
- Flash message support in base template
- Test config update for email service dev mode
- Comprehensive test suite with 8 tests

All tests passing (86 total), 91% coverage maintained.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

tests/unit/test_email_service.py

@@ -15,6 +15,7 @@ class TestEmailService:
         """Test EmailService initialization with API key."""
         with app.app_context():
             app.config["RESEND_API_KEY"] = "test-api-key"
+            app.config["FLASK_ENV"] = "production"
             service = EmailService()
             assert service.api_key == "test-api-key"
             assert service.dev_mode is False