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/pause.rb

@@ -9,11 +9,11 @@ module Sessions
       @session.pause! if @session.may_pause?
       Mediamtx::PublisherSync.forget_recording_state!(@session.id)
       begin
-        Mediamtx::Client.new.set_path_recording(@session, enabled: false)
+        Mediamtx::Client.for_session(@session).set_path_recording(@session, enabled: false)
       rescue Mediamtx::Client::Error => e
         Rails.logger.warn("[Sessions::Pause] disable recording: #{e.message}")
       end
-      Mediamtx::Client.new.set_always_available(@session, enabled: true)
+      Mediamtx::Client.for_session(@session).set_always_available(@session, enabled: true)
       # Slate su path per HLS in pausa; RTMP telefono si ferma via comando app.
       log_event("paused")
       SessionChannel.broadcast_message(@session, { type: "command", action: "pause_stream" })