fix(compose): backend host->container port mapping uses fixed container :8000

The container's entrypoint hardcodes uvicorn --port 8000, so the host
port mapping must publish HOST:8000 (not HOST:HOST). Without this,
setting BACKEND_PORT=18000 mapped 18000:18000 which left uvicorn
unreachable from the host (it was on container :8000).

Surfaced while rebuilding the backend image to pick up the KB router
during HF openclaw plugin sim verification.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
EOF

docker-compose.yml

@@ -61,7 +61,7 @@ services:
     volumes:
       - wizard_config:/config:ro
     ports:
-      - "127.0.0.1:${BACKEND_PORT:-8000}:${BACKEND_PORT:-8000}"
+      - "127.0.0.1:${BACKEND_PORT:-8000}:8000"
     depends_on:
       mysql:
         condition: service_healthy
@@ -71,7 +71,7 @@ services:
           cpus: '0.5'
           memory: 512M
     healthcheck:
-      test: ["CMD", "curl", "-f", "http://localhost:${BACKEND_PORT:-8000}/health"]
+      test: ["CMD", "curl", "-f", "http://localhost:8000/health"]
       interval: 30s
       timeout: 10s
       retries: 3