Corregge validazione upload copertina e generazione slate al go-live.

Rifiuta file troppo grandi o non-immagine via Marcel prima di attach, e genera la slate in sync anche quando cover_image esiste ma cover_slate non è ancora pronta.

Co-authored-by: Cursor <cursoragent@cursor.com>

backend/app/controllers/api/v1/base_controller.rb

@@ -2,6 +2,7 @@ module Api
   module V1
     class BaseController < ApplicationController
       rescue_from Teams::EntitlementError, with: :render_entitlement_error
+      rescue_from BrandingAttachments::CoverUploadError, with: :render_cover_upload_error
       rescue_from Youtube::BroadcastService::Error, with: :render_youtube_error
 
       private
@@ -20,6 +21,10 @@ module Api
           billing_url: error.billing_url
         }, status: :forbidden
       end
+
+      def render_cover_upload_error(error)
+        render json: { error: error.message, error_code: "cover_upload_invalid" }, status: :unprocessable_entity
+      end
     end
   end
 end