Corregge cloud-init montato, PublisherOnline multi-nodo e E2E locale-aware.

Senza volume stream-node i nodi Hetzner nascevano senza MediaMTX; sync live usa ora Client.for_session e rtmpconns (MediaMTX 1.20).

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

infra/stream-node/cloud-init.yaml

@@ -49,10 +49,11 @@ write_files:
       docker pull bluenviron/mediamtx:latest
       docker rm -f mediamtx 2>/dev/null || true
       # Slate offline (alwaysAvailable) — richiesta da Mediamtx::Client#create_path
+      # MediaMTX 1.20+: il publisher RTMP deve matchare canali audio della slate (stereo).
       mkdir -p /slates
       if [ ! -f /slates/offline.mp4 ]; then
         ffmpeg -y -f lavfi -i color=c=black:s=1280x720:d=2 -f lavfi -i anullsrc=r=44100:cl=stereo \
-          -c:v libx264 -t 2 -pix_fmt yuv420p -c:a aac -shortest /slates/offline.mp4
+          -c:v libx264 -t 2 -pix_fmt yuv420p -c:a aac -ac 2 -shortest /slates/offline.mp4
       fi
       # Debian cloud image: docker.io senza apparmor_parser → serve unconfined o pkg apparmor
       docker run -d --name mediamtx --restart unless-stopped --network host \