chore(neuron): log load_model failures server-side with full chain

The HTTP handler now emits a tracing::warn on load_model failures with
the expanded anyhow chain (format!("{e:#}")) before returning the 400.
journalctl -u neuron will surface the underlying hf-hub /
materialisation error without needing to capture the curl response
body separately.

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

crates/neuron/src/api.rs

@@ -69,11 +69,22 @@ async fn load_model(
     let registry = state.registry.read().await;
     match registry.load_model(&spec).await {
         Ok(()) => Json(json!({"status": "loaded"})).into_response(),
-        Err(e) => (
+        Err(e) => {
+            // Log the full anyhow chain server-side so journalctl shows
+            // the underlying failure (hf-hub timeout, permission denied,
+            // disk full, etc.) without needing to inspect the HTTP
+            // response body separately.
+            tracing::warn!(
+                model = %spec.model_id,
+                error = %format!("{e:#}"),
+                "load_model failed"
+            );
+            (
                 StatusCode::BAD_REQUEST,
                 Json(json!({"error": format!("{e:#}")})),
             )
-            .into_response(),
+                .into_response()
+        }
     }
 }