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/test_helper.rb

@@ -19,6 +19,14 @@ module ActiveSupport
 
     # Add more helper methods to be used by all tests here...
 
+    # Mock Stripe for tests
+    setup do
+      # Mock Stripe checkout session creation
+      Stripe::Checkout::Session.stubs(:create).returns(
+        Struct.new(:id, :url).new("cs_test_session", "https://checkout.stripe.com/test")
+      )
+    end
+
     # Helper to create users with completed onboarding by default for tests
     def create_test_user(attributes = {})
       User.create!({