fix: disable email transcript for free plans (#13509)

- Block email transcript functionality for accounts without a paid plan
to prevent SES abuse.

app/controllers/api/v1/accounts/conversations_controller.rb

@@ -70,6 +70,7 @@ class Api::V1::Accounts::ConversationsController < Api::V1::Accounts::BaseContro
 
   def transcript
     render json: { error: 'email param missing' }, status: :unprocessable_entity and return if params[:email].blank?
+    return render_payment_required('Email transcript is not available on your plan') unless @conversation.account.email_transcript_enabled?
     return head :too_many_requests unless @conversation.account.within_email_rate_limit?
 
     ConversationReplyMailer.with(account: @conversation.account).conversation_transcript(@conversation, params[:email])&.deliver_later