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/mqtt.py

@@ -0,0 +1,54 @@
+"""MQTT router — exposes MQTT entity state and publish endpoint."""
+
+from __future__ import annotations
+
+import logging
+from typing import Any, Dict, Optional
+
+from fastapi import APIRouter, HTTPException
+from pydantic import BaseModel
+
+from server.services.mqtt_service import mqtt_service
+
+logger = logging.getLogger(__name__)
+
+router = APIRouter(prefix="/api/mqtt", tags=["mqtt"])
+
+
+class PublishRequest(BaseModel):
+    topic: str
+    payload: Any
+    retain: bool = False
+
+
+@router.get("")
+async def get_mqtt_state() -> Dict[str, Any]:
+    """Return all stored MQTT entities."""
+    return {
+        "connected": mqtt_service.connected,
+        "entities": mqtt_service.get_entities(),
+        "topic_count": len(mqtt_service.store),
+    }
+
+
+@router.get("/topic/{topic:path}")
+async def get_mqtt_topic(topic: str) -> Dict[str, Any]:
+    """Return value for a specific MQTT topic."""
+    msg = mqtt_service.store.get(topic)
+    if not msg:
+        raise HTTPException(status_code=404, detail=f"Topic '{topic}' not found")
+    return {
+        "topic": msg.topic,
+        "value": msg.payload,
+        "timestamp": msg.timestamp,
+    }
+
+
+@router.post("/publish")
+async def publish_mqtt(req: PublishRequest) -> Dict[str, str]:
+    """Publish a message to the MQTT broker."""
+    try:
+        await mqtt_service.publish(req.topic, req.payload, retain=req.retain)
+        return {"status": "ok", "topic": req.topic}
+    except RuntimeError as exc:
+        raise HTTPException(status_code=503, detail=str(exc))