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/events/show.html.erb

@@ -10,9 +10,9 @@
     <!-- Event main wrapper -->
     <div class="bg-white rounded-2xl shadow-xl overflow-hidden">
       <!-- Event Header with Image -->
-      <% if @event.image.present? %>
+      <% if @event.image.attached? %>
         <div class="relative h-96">
-          <%= image_tag @event.image, class: "w-full h-full object-cover" %>
+          <%= image_tag @event.event_image_variant(:large), class: "w-full h-full object-cover" %>
           <div class="absolute inset-0 bg-gradient-to-t from-black via-black/70 to-transparent"></div>
           <div class="absolute bottom-0 left-0 right-0 p-6 md:p-8">
             <div class="max-w-4xl mx-auto">