Aggiunge registry nodi stream e provisioning Hetzner/lab per lo scale-out.

Prepara l'architettura multi-nodo (MediaMTX+ffmpeg) con assignment URL per sessione, admin di provision/drain e provider Cloud/DNS astratti verso mltv-stream.net.

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

backend/config/routes.rb

@@ -110,6 +110,11 @@ Rails.application.routes.draw do
         post :regia_link
       end
     end
+    resources :stream_nodes, only: %i[index create destroy] do
+      member do
+        post :drain
+      end
+    end
     get "youtube/platform", to: "youtube#platform", as: :youtube_platform
   end