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/controllers/promoter/events_controller.rb

@@ -29,6 +29,8 @@ class Promoter::EventsController < ApplicationController
     if @event.save
       redirect_to promoter_event_path(@event), notice: "Event créé avec succès!"
     else
+      # If validation fails and an image was attached, purge it
+      @event.image.purge if @event.image.attached?
       render :new, status: :unprocessable_entity
     end
   end