fix(neuron): surface full anyhow chain + ensure $HOME exists at start

Two fixes uncovered by the live validation against beast/benjy/quadbrat:

1. api.rs swallowed everything beyond the outermost anyhow context.
   The validation script reported '{"error":"fetch GGUF ...gguf"}' but
   the actual underlying hf-hub failure (cache dir creation, network,
   auth, etc.) was hidden. Switching every error response to
   format!("{e:#}") expands the full cause chain via anyhow's
   alternate Display format.

2. The neuron systemd unit declared the service user but never ensured
   /var/lib/neuron (its $HOME) existed. hf-hub defaults its cache to
   ~/.cache/huggingface/hub — when $HOME is absent the cache dir
   creation fails and the download aborts. Adding `StateDirectory=neuron`
   makes systemd create + chown that directory at activation; no spec
   change needed.

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

data/neuron.service

@@ -10,6 +10,12 @@ Restart=on-failure
 RestartSec=5
 User=neuron
 Group=neuron
+# /var/lib/neuron is the neuron user's $HOME — hf-hub writes its
+# default cache there (~/.cache/huggingface/hub). Without this directive
+# systemd doesn't create the directory and hf-hub downloads fail with
+# "fetch GGUF <file>: failed to create cache dir".
+StateDirectory=neuron
+StateDirectoryMode=0755
 # Loading default_models from neuron.toml happens before the HTTP
 # listener binds; large models can take many minutes to download and
 # materialise on first activation. systemd's default TimeoutStartSec