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/jobs/cleanup_expired_sessions_job.rb

@@ -6,7 +6,7 @@ class CleanupExpiredSessionsJob
                  .where("updated_at < ?", 6.hours.ago)
                  .find_each do |session|
       session.fail! if session.may_fail?
-      Mediamtx::Client.new.delete_path(session)
+      Mediamtx::Client.for_session(session).delete_path(session)
     end
   end
 end