feat: Complete hybrid image upload system with URL compatibility

- Add hybrid image system supporting both file uploads and URL images
- Implement Active Storage for file uploads while preserving existing URL functionality
- Update Event model with both has_one_attached :image and image_url virtual attribute
- Create tabbed interface in event forms for upload/URL selection
- Add JavaScript preview functionality for both upload and URL inputs
- Fix promotion code validation issue in tests using distinct() to prevent duplicates
- Update all views to use hybrid display methods prioritizing uploads over URLs
- Update seeds file to use image_url attribute for compatibility
- Ensure backward compatibility with existing events using URL images

🤖 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 +1,13 @@
 <%= 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.event_image_variant(:small), alt: event.name, class: "w-full h-full object-cover" if event.image.attached? %>
+      <% if event.has_image? %>
+        <% if event.image.attached? %>
+          <%= image_tag event.event_image_variant(:small), alt: event.name, class: "w-full h-full object-cover" %>
+        <% else %>
+          <%= image_tag event.image_url, alt: event.name, class: "w-full h-full object-cover" %>
+        <% end %>
+      <% end %>
     </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">