The engine-side "Waiting for model reply..." content injection ran inside the
GIL-blocked engine handler during model load, so it never streamed live -- it
only got concatenated into the final reply and polluted the output. Removed it.

Load-status now lives in the responsive front-proxy/broker layer:
- SSE: broker_execute_stream yields a NON-content chunk (empty delta.content +
  x_queue_info status) during each not-ready wait (engine down / starting /
  model loading), so a watching client sees progress without reply pollution.
- out-of-band: the existing broker `pending` keepalives are kept.

Both are gated by a single flag, resolved by _load_status_enabled(model):
- global models.load_status_updates (default True)
- per-model "load_status_updates" override in models.json (unset = inherit)
The same gate drives the SSE side and, via client.status_gate, the out-of-band
`pending` keepalives. Unknown model defaults to on so the relay deadline stays
protected.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01RdMufYvtTbtGDWsiZVoXce