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/db/migrate/20260602120000_add_pending_plan_to_subscriptions.rb

@@ -0,0 +1,9 @@
+class AddPendingPlanToSubscriptions < ActiveRecord::Migration[7.2]
+  def change
+    change_table :subscriptions, bulk: true do |t|
+      t.references :pending_plan, type: :uuid, foreign_key: { to_table: :plans }
+      t.string :pending_billing_interval
+      t.string :stripe_schedule_id
+    end
+  end
+end