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/pages/home.html.erb

@@ -89,10 +89,8 @@
               <div class="bg-white rounded-2xl shadow-lg hover:shadow-2xl transition-all duration-300 overflow-hidden">
                 <!-- Event Image -->
                 <div class="relative overflow-hidden aspect-[4/3]">
-                  <% if event.image.present? %>
-                    <img src="<%= event.image %>"
-                         alt="<%= event.name %>"
-                         class="w-full h-full object-cover group-hover:scale-105 transition-transform duration-300">
+                  <% if event.image.attached? %>
+                    <%= image_tag event.event_image_variant(:medium), alt: event.name, class: "w-full h-full object-cover group-hover:scale-105 transition-transform duration-300" %>
                   <% else %>
                     <div class="w-full h-full bg-gradient-to-br from-purple-600 to-blue-600 flex items-center justify-center">
                       <i data-lucide="calendar" class="w-16 h-16 text-white"></i>