fix(decklink): mount /dev/blackmagic in sidecar + remote node routing via node-agent

Two bugs fixed:
1. SDI capture sidecar never had /dev/blackmagic bound — ffmpeg opened the
   decklink input inside a container with no device nodes, so frame=0.
   Fix: local spawns now push '/dev/blackmagic:/dev/blackmagic' onto Binds
   when source_type='sdi'.

2. recorders.js always spawned sidecars against the local Docker socket
   (zampp1), even when a recorder's node_id pointed at zampp2 (where the
   card is). Fix: resolveNodeTarget() looks up the recorder's cluster node;
   if it's a different hostname the sidecar is spawned via a new
   POST /sidecar/start endpoint on the remote node-agent.

node-agent gains three new routes (all talk to the local Docker socket):
  POST   /sidecar/start         — create + start container (host network,
                                   privileged, /dev/blackmagic bind for sdi)
  DELETE /sidecar/:id           — stop + remove
  GET    /sidecar/:id/status    — inspect + poll capture service

docker-compose.worker.yml: add /var/run/docker.sock and LIVE_DIR to
node-agent so it can spawn sidecars, and document build-capture prerequisite.: docker-compose.worker.yml

docker-compose.worker.yml

@@ -17,6 +17,7 @@
 #   REDIS_URL, DATABASE_URL, S3_ENDPOINT, S3_BUCKET, S3_ACCESS_KEY, S3_SECRET_KEY
 #   BMD_DEVICE_0  DeckLink device path (default: /dev/blackmagic/dv0)
 #   BMD_DEVICE_1  DeckLink device path (default: /dev/blackmagic/dv1)
+#   LIVE_DIR      Host path for HLS live segments (default: /mnt/NVME/MAM/wild-dragon-live)
 #
 # Profiles:
 #   (default)           node-agent only — cluster visibility + hardware heartbeat
@@ -25,6 +26,11 @@
 #
 # To enable GPU transcoding, also apply docker-compose.gpu.yml:
 #   docker compose -f docker-compose.worker.yml -f docker-compose.gpu.yml --profile worker up -d
+#
+# NOTE: The node-agent mounts /var/run/docker.sock to spawn on-demand SDI
+# capture sidecars when the primary mam-api routes a recorder to this node.
+# Build the capture image before first use:
+#   docker compose -f docker-compose.worker.yml build capture
 
 services:
 
@@ -45,10 +51,12 @@ services:
       GPU_COUNT:     ${GPU_COUNT:--1}
       BMD_COUNT:     ${BMD_COUNT:--1}
       BMD_MODEL:     ${BMD_MODEL:-}
+      LIVE_DIR:      ${LIVE_DIR:-/mnt/NVME/MAM/wild-dragon-live}
+    volumes:
+      - /var/run/docker.sock:/var/run/docker.sock
+      - /dev:/dev:ro
     devices:
       - /dev/blackmagic:/dev/blackmagic
-    volumes:
-      - /dev:/dev:ro
 
   worker:
     build: ./services/worker