Billing upgrade/downgrade, inviti in app e diretta YouTube da mobile.

Upgrade Stripe con addebito immediato; downgrade programmato a fine periodo.
UX billing più sobria con box info; icone piani. API inviti e OAuth YouTube
per staff trasmissione; deep link join; scelta partita programmata o nuova.

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

backend/app/models/subscription.rb

@@ -4,6 +4,7 @@ class Subscription < ApplicationRecord
 
   belongs_to :club
   belongs_to :plan
+  belongs_to :pending_plan, class_name: "Plan", optional: true
 
   validates :status, inclusion: { in: STATUSES }
   validates :club_id, uniqueness: true
@@ -25,4 +26,8 @@ class Subscription < ApplicationRecord
   def free?
     plan.slug == "free"
   end
+
+  def plan_change_pending?
+    pending_plan_id.present?
+  end
 end