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>

db/schema.rb

@@ -56,7 +56,6 @@ ActiveRecord::Schema[8.0].define(version: 2025_09_29_222616) do
     t.boolean "allow_booking_during_event", default: false, null: false
     t.datetime "created_at", null: false
     t.datetime "updated_at", null: false
-    t.string "image_url"
     t.index ["featured"], name: "index_events_on_featured"
     t.index ["latitude", "longitude"], name: "index_events_on_latitude_and_longitude"
     t.index ["state"], name: "index_events_on_state"
@@ -159,7 +158,6 @@ ActiveRecord::Schema[8.0].define(version: 2025_09_29_222616) do
     t.index ["reset_password_token"], name: "index_users_on_reset_password_token", unique: true
   end
 
-  add_foreign_key "active_storage_attachments", "active_storage_blobs", column: "blob_id"
   add_foreign_key "active_storage_variant_records", "active_storage_blobs", column: "blob_id"
   add_foreign_key "order_promotion_codes", "orders"
   add_foreign_key "order_promotion_codes", "promotion_codes"