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/promoter/events/show.html.erb

@@ -174,9 +174,9 @@
     <!-- Main content -->
     <div class="lg:col-span-2 space-y-6 lg:space-y-8">
       <!-- Event image -->
-      <% if @event.image.present? %>
+      <% if @event.image.attached? %>
         <div class="aspect-video bg-gray-100 rounded-2xl overflow-hidden">
-          <img src="<%= @event.image %>" alt="<%= @event.name %>" class="w-full h-full object-cover">
+          <%= image_tag @event.event_image_variant(:large), alt: @event.name, class: "w-full h-full object-cover" %>
         </div>
       <% end %>