fix: remove MQTT_BROKER env override — let config.json mqttSources connect to external brokers

The env var was overriding the config and forcing Go staging to only
connect to its own empty local mosquitto, missing all external data.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

docker-compose.yml

@@ -64,7 +64,6 @@ services:
       - caddy-data-staging-go:/data/caddy
     environment:
       - NODE_ENV=staging
-      - MQTT_BROKER=mqtt://localhost:1883
     healthcheck:
       test: ["CMD", "wget", "-qO-", "http://localhost:3000/api/stats"]
       interval: 30s