Fix MQTT public broker URL not being passed to Docker container

The MQTT_PUBLIC_BROKER_URL environment variable was defined in .env but
wasn't being passed to the app container in docker-compose.yml, causing
emails to still show the internal broker URL (mqtt://mosquitto:1883)
instead of the public one (mqtt://192.168.10.118:1883).

Changes:
- Added MQTT_PUBLIC_BROKER_URL to docker-compose.yml environment section
- Fixed .env to use proper format: mqtt://192.168.10.118:1883

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

docker-compose.yml

@@ -13,6 +13,7 @@ services:
       - NEXTAUTH_URL=${NEXTAUTH_URL:-http://localhost:3000}
       - NEXTAUTH_SECRET=${NEXTAUTH_SECRET}
       - MQTT_BROKER_URL=${MQTT_BROKER_URL:-mqtt://mosquitto:1883}
+      - MQTT_PUBLIC_BROKER_URL=${MQTT_PUBLIC_BROKER_URL}
       - MQTT_USERNAME=${MQTT_ADMIN_USERNAME:-admin}
       - MQTT_PASSWORD=${MQTT_ADMIN_PASSWORD:-admin}
       - MOSQUITTO_PASSWORD_FILE=/mosquitto/config/password.txt