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/plan_change_kind.rb

@@ -0,0 +1,33 @@
+module Billing
+  module Stripe
+    module PlanChangeKind
+      module_function
+
+      def classify(from_plan_slug:, from_interval:, to_plan_slug:, to_interval:)
+        from_tier = Plan.tier(from_plan_slug)
+        to_tier = Plan.tier(to_plan_slug)
+        from_interval = PriceCatalog.normalize_interval(from_interval)
+        to_interval = PriceCatalog.normalize_interval(to_interval)
+
+        return :upgrade if to_tier > from_tier
+        return :downgrade if to_tier < from_tier
+
+        if from_interval == to_interval
+          :same
+        elsif to_interval == "yearly"
+          :upgrade
+        else
+          :downgrade
+        end
+      end
+
+      def upgrade?(**kwargs)
+        classify(**kwargs) == :upgrade
+      end
+
+      def downgrade?(**kwargs)
+        classify(**kwargs) == :downgrade
+      end
+    end
+  end
+end