docs(broker): establish remote Docker deployment plan for nats-server with networking/security guides and add D-15~D-21 freshness guards

.mam.env.example

@@ -80,6 +80,10 @@
 #default: hermes
 # MQTT_CLIENT_ID_PREFIX=hermes
 
+# MQTT keepalive interval (seconds). Used by paho-mqtt client connections.
+#default: 60
+# MQTT_KEEPALIVE=60
+
 # Log level for MAM runtime components (DEBUG, INFO, WARN, ERROR).
 #default: INFO
 # MAM_LOG_LEVEL=INFO