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/services/billing/stripe/change_plan_result.rb

@@ -0,0 +1,22 @@
+module Billing
+  module Stripe
+    ChangePlanResult = Struct.new(
+      :kind,
+      :plan_slug,
+      :interval,
+      :effective_at,
+      :amount_cents,
+      :currency,
+      :invoice_id,
+      keyword_init: true
+    ) do
+      def upgrade?
+        kind == :upgrade
+      end
+
+      def downgrade?
+        kind == :downgrade
+      end
+    end
+  end
+end