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/config/routes.rb

@@ -11,6 +11,9 @@ Rails.application.routes.draw do
       post "auth/refresh", to: "auth#refresh"
       get "auth/me", to: "auth#me"
 
+      get "invitations/:token", to: "invitations#show"
+      post "invitations/:token/accept", to: "invitations#accept"
+
       resources :teams do
         member do
           post :members, action: :add_member