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/views/components/_event_item.html.erb

@@ -1,7 +1,7 @@
 <%= link_to event_path(event.slug, event), class: "group block p-4 rounded-lg border border-slate-200 dark:border-slate-700 hover:border-purple-300 dark:hover:border-purple-600 hover:shadow-md transition-all duration-200" do %>
   <div class="flex items-center space-x-4">
     <div class="w-16 h-16 bg-slate-200 dark:bg-slate-700 rounded-lg overflow-hidden flex-shrink-0">
-      <%= image_tag event.image, alt: event.name, class: "w-full h-full object-cover" if event.image.present? %>
+      <%= image_tag event.event_image_variant(:small), alt: event.name, class: "w-full h-full object-cover" if event.image.attached? %>
     </div>
     <div class="flex-1 min-w-0">
       <h3 class="text-lg font-semibold text-slate-900 dark:text-slate-100 group-hover:text-purple-600 dark:group-hover:text-purple-400 transition-colors duration-200">