derive an agent's backend hosts into the egress allow list at the controller

swarm-controller/src/backend_env.rs

@@ -76,6 +76,8 @@ pub async fn put_backend_env(
         error_problem(StatusCode::INTERNAL_SERVER_ERROR, &e.to_string())
     })?;
 
+    crate::backend_egress::follow_env(&state, &agent, Some(&stored.env)).await?;
+
     tracing::info!(%hive, %agent, "backend env stored");
     Ok(StatusCode::NO_CONTENT)
 }