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/index.html.erb

@@ -22,13 +22,9 @@
       <% @events.each do |event| %>
         <article class="group bg-white rounded-2xl shadow-lg hover:shadow-2xl transition-all duration-300 overflow-hidden transform hover:-translate-y-1">
           <%= link_to event_path(event.slug, event), class: "block" do %>
-            <% if event.image.present? %>
+            <% if event.image.attached? %>
               <div class="relative overflow-hidden aspect-[4/3]">
-                <img
-                  src="<%= event.image %>"
-                  alt="<%= event.name %>"
-                  class="w-full h-full object-cover group-hover:scale-105 transition-transform duration-300"
-                >
+                <%= 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" %>
                 <!-- Event featured badge -->
                 <% if event.featured? %>
                   <div class="absolute top-4 left-4">