Replace browser MQTT bridge with backend device-events service

The frontend previously connected directly to the MQTT broker via a
CDN-loaded Paho client and a Paho-specific ws proxy. Move that
responsibility into the backend: DeviceDataService persists MQTT
messages to Postgres, and DeviceEventsController exposes them over a
REST endpoint plus a WebSocket gateway that the frontend now consumes
directly. Also adds a pgadmin service for inspecting the database.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

docker-compose.yml

@@ -55,5 +55,15 @@ services:
       NODE_ENV: development
     command: npm run dev
 
+  pgadmin:
+    image: dpage/pgadmin4:8
+    environment:
+      PGADMIN_DEFAULT_EMAIL: admin@example.com
+      PGADMIN_DEFAULT_PASSWORD: admin
+    ports:
+      - "5050:80"
+    depends_on:
+      - postgres
+
 volumes:
   postgres-data: