feat: add MQTT integration for real-time entity updates

- aiomqtt async client with auto-reconnect and topic store
- MQTT router: GET /api/mqtt, GET /api/mqtt/topic/{path}, POST /api/mqtt/publish
- MQTT entities included in /api/all + WebSocket broadcast
- MqttCard frontend component with category filters, entity list
- Configurable via ENV: MQTT_HOST, MQTT_PORT, MQTT_USERNAME,
  MQTT_PASSWORD, MQTT_TOPICS (comma-separated or JSON array)
- Gracefully disabled when MQTT_HOST is not set

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

server/routers/dashboard.py

@@ -15,6 +15,7 @@ from server.routers.news import get_news_articles
 from server.routers.servers import get_servers
 from server.routers.tasks import get_tasks
 from server.routers.weather import get_weather
+from server.services.mqtt_service import mqtt_service
 
 logger = logging.getLogger(__name__)
 
@@ -58,6 +59,10 @@ async def get_all() -> Dict[str, Any]:
         "servers": servers_data,
         "ha": ha_data,
         "tasks": tasks_data,
+        "mqtt": {
+            "connected": mqtt_service.connected,
+            "entities": mqtt_service.get_entities(),
+        },
         "timestamp": datetime.now(timezone.utc).isoformat(),
     }