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/controllers/api/v1/stream_sessions_controller.rb

@@ -180,6 +180,7 @@ module Api
           platform: session.platform,
           rtmp_ingest_url: session.rtmp_ingest_url,
           hls_playback_url: session.hls_playback_url,
+          stream_node: session.stream_node&.slug,
           watch_page_url: session.matchlivetv_platform? ? session.watch_page_url : nil,
           share_url: session.share_url,
           youtube_watch_url: session.youtube_watch_url,