feat: Implement event image upload system for promoters

- Add Active Storage migrations for file attachments
- Update Event model to handle image uploads with validation
- Replace image URL fields with file upload in forms
- Add client-side image preview with validation
- Update all views to display uploaded images properly
- Fix JSON serialization to prevent stack overflow in API
- Add custom image validation methods for format and size
- Include image processing variants for different display sizes
- Fix promotion code test infrastructure and Stripe configuration

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

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

test/fixtures/users.yml

@@ -5,6 +5,7 @@ one:
   encrypted_password: <%= Devise::Encryptor.digest(User, 'password123') %>
   last_name: Trump
   first_name: Donald
+  is_professionnal: true
   onboarding_completed: true
 
 two: