Document backup pipeline behaviour

bin/mqtt-trigger-listener.sh

@@ -1,5 +1,7 @@
 #!/usr/bin/env bash
-# Listen for an authenticated JSON command and run the backup pipeline.
+# Long-running MQTT command listener, normally managed by systemd as root.
+# Accepts only {"action":"run"}; broker authentication and ACLs should restrict
+# who can publish to MQTT_COMMAND_TOPIC.
 set -Eeuo pipefail
 SCRIPT_DIR="$(cd -- "$(dirname -- "${BASH_SOURCE[0]}")/.." && pwd -P)"
 . "$SCRIPT_DIR/lib/common.sh"
@@ -20,5 +22,7 @@ mosquitto_sub "${MQTT_ARGS[@]}" -R -t "$MQTT_COMMAND_TOPIC" | while IFS= read -r
     continue
   fi
   log MQTT "Accepted backup run command"
+  # backup.sh owns concurrency control. A command received during an active run
+  # fails safely on the same flock used by cron and manual invocations.
   "$SCRIPT_DIR/backup.sh" || log ERROR "MQTT-triggered backup failed"
 done