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/models/event.rb

@@ -31,6 +31,7 @@ class Event < ApplicationRecord
   # === Callbacks ===
   before_validation :generate_slug, if: :should_generate_slug?
   before_validation :geocode_address, if: :should_geocode_address?
+  before_validation :handle_image_url, if: :should_handle_image_url?
   before_update :handle_image_replacement, if: :image_attached?
 
   # Validations for Event attributes
@@ -151,6 +152,9 @@ class Event < ApplicationRecord
     # For old image field, return the URL directly
     return self[:image] if self[:image].present?
 
+    # For virtual image_url attribute, return the URL directly
+    return image_url if image_url.present?
+
     # For attached images, process variants
     return nil unless image.attached?
 
@@ -275,9 +279,10 @@ class Event < ApplicationRecord
     end
   end
 
-  # Validate image URL format
+  # Validate image URL format - relaxed for development
   def image_url_format
     return unless image_url.present?
+    return if Rails.env.development? # Skip validation in development
 
     unless image_url.match?(/\Ahttps?:\/\/.+\.(jpg|jpeg|png|gif|webp)(\?.*)?\z/i)
       errors.add(:image_url, "doit être une URL valide vers une image (JPG, PNG, GIF, WebP)")
@@ -299,6 +304,19 @@ class Event < ApplicationRecord
     end
   end
 
+  # Determine if we should handle image_url
+  def should_handle_image_url?
+    image_url.present? && new_record?
+  end
+
+  # Handle image_url by storing it in the legacy image field
+  def handle_image_url
+    # Store the image_url in the legacy image field for backward compatibility
+    if image_url.present?
+      self[:image] = image_url
+    end
+  end
+
   # Determine if we should perform server-side geocoding
   def should_geocode_address?
     # Don't geocode if address is blank