fix: Update Event model to handle image URLs and fix image display

- Relax image URL validation in development environment
- Add callback to store image_url in legacy image field for compatibility
- Fix event_image_variant method to handle virtual image_url attribute
- Simplify event show view to use unified image display method
- Fix seeds slug reference for "La belle époque" event

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

app/views/events/show.html.erb

@@ -12,11 +12,7 @@
       <!-- Event Header with Image -->
       <% if @event.has_image? %>
         <div class="relative h-96">
-          <% if @event.image.attached? %>
-            <%= image_tag @event.event_image_variant(:large), class: "w-full h-full object-cover" %>
-          <% else %>
-            <%= image_tag @event.image_url, class: "w-full h-full object-cover", alt: @event.name %>
-          <% end %>
+          <%= image_tag @event.event_image_variant(:large), class: "w-full h-full object-cover", alt: @event.name %>
           <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">