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>

app/controllers/orders_controller.rb

@@ -164,6 +164,8 @@ class OrdersController < ApplicationController
         flash[:alert] = "Erreur lors de la création de la session de paiement"
       end
     end
+
+    render :checkout
   end
 
   # Increment payment attempt - called via AJAX when user clicks pay button