fix: add DNS config to demo backend container for API access

Explicit Google/Cloudflare DNS prevents resolution failures when
the Docker host's DNS is unavailable or misconfigured.

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

docker-compose.demo.yml

@@ -16,6 +16,9 @@ services:
       ANTHROPIC_API_KEY: ${ANTHROPIC_API_KEY:-}
     expose:
       - "5959"
+    dns:
+      - 8.8.8.8
+      - 1.1.1.1
     restart: unless-stopped
 
   neode-web: