# US-023 — Heartbeat stats payload: request counters + dynamic reassignment response

Status: done
Priority: Medium
Stage: Implemented

## Context

Node heartbeats are currently empty POSTs. The tracker has no visibility into per-node load,
making load balancing and assignment decisions blind. Heartbeats should carry cumulative stats.
The heartbeat response channel is also the natural place for the tracker to deliver reassignment
instructions without requiring a node restart.

## Acceptance criteria

- [ ] Heartbeat POST body includes: `total_requests`, `failed_requests`, `queue_depth`, `uptime_seconds`, `status`
- [ ] `TorchNodeServer` tracks the three counters with a `threading.Lock`
- [ ] Tracker stores the last heartbeat payload per node
- [ ] Heartbeat response may include `new_assignment: {model, shard_start, shard_end}`; node logs it
- [ ] Stats survive tracker outage: buffered locally, flushed on next successful heartbeat
- [ ] `python -m pytest` passes

## Notes

Hot-reload (loading a new shard without restart) is deferred to a future story. The response
field is wired so trackers can send the signal; nodes log it but don't act yet.