fix: route Claude API through backend instead of nginx envsubst

- Add Claude API proxy in mock-backend.js (reads ANTHROPIC_API_KEY from env)
- Supports SSE streaming via pipe
- Move ANTHROPIC_API_KEY to backend service in docker-compose.demo.yml
- Remove envsubst from entrypoint (no longer needed)
- nginx-demo.conf proxies /aiui/api/claude/ to backend

This fixes the 401 error when Portainer doesn't pass env vars to
nginx correctly — the Node.js backend reads process.env directly.

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

docker-compose.demo.yml

@@ -13,6 +13,7 @@ services:
     container_name: archy-demo-backend
     environment:
       VITE_DEV_MODE: "existing"  # Skip setup/onboarding, go straight to login
+      ANTHROPIC_API_KEY: ${ANTHROPIC_API_KEY:-}
     expose:
       - "5959"
     restart: unless-stopped
@@ -22,8 +23,6 @@ services:
       context: .
       dockerfile: neode-ui/Dockerfile.web
     container_name: archy-demo-web
-    environment:
-      ANTHROPIC_API_KEY: ${ANTHROPIC_API_KEY:-}
     ports:
       - "4848:80"
     depends_on: