fix(serving): wire MLflow auth and Host header for container-to-container calls

- Pass MLFLOW_ADMIN_PASSWORD as fallback password credential
- Set host_header='localhost' to satisfy MLflow's --allowed-hosts check
  (MLflow rejects Host: mlflow but accepts Host: localhost)
- Default MLFLOW_TRACKING_URI to http://mlflow:5000 in compose so the
  env_file value is not silently overridden to empty

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

infra/docker/docker-compose.yml

@@ -71,7 +71,7 @@ services:
     environment:
       LITELLM_URL: ${LITELLM_URL:-http://host.docker.internal:4000}
       OLLAMA_URL: ${OLLAMA_URL:-http://host.docker.internal:11434}
-      MLFLOW_TRACKING_URI: ${MLFLOW_TRACKING_URI:-}
+      MLFLOW_TRACKING_URI: ${MLFLOW_TRACKING_URI:-http://mlflow:5000}
     extra_hosts:
       - "host.docker.internal:host-gateway"
     ports: