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/app/services/sessions/stop.rb

@@ -33,7 +33,7 @@ module Sessions
     end
 
     def remove_mediamtx_paths!
-      Mediamtx::Client.new.delete_path(@session)
+      Mediamtx::Client.for_session(@session).delete_path(@session)
     rescue Mediamtx::Client::Error => e
       Rails.logger.warn("[Sessions::Stop] delete_path #{@session.id}: #{e.message}")
     end