# US-025 — Model usage statistics: rolling RPM windows + SQLite persistence

Status: done
Priority: Medium
Stage: Implemented

## Context

Trackers need per-model request-rate data to drive smart shard assignment (US-026) and
to give operators visibility into what the network is actually serving. Stats must survive
tracker restarts and should be shareable across a tracker cluster.

## Design

- `_RollingCounter`: circular-bucket counter, epoch-indexed, stale buckets auto-reset
- Three windows: 60×1-min buckets (last hour), 24×1-hr (last day), 30×1-day (last month)
- `_StatsCollector`: `record_request()`, `get_local_rpms()`, `merge_peer_rpms()`, `get_combined_stats()`
- SQLite persistence via `--stats-db PATH`
- Gossip: each tracker keeps its own slice; merge is additive (not averaged)

## Acceptance criteria

- [ ] `_RollingCounter` passes unit tests (record, rpm, stale bucket reset)
- [ ] `_StatsCollector` accumulates and merges peer slices additively
- [ ] SQLite round-trip: buckets saved and restored across restart
- [ ] `GET /v1/stats` returns combined stats JSON
- [ ] `POST /v1/stats/gossip` accepts peer slice and merges
- [ ] `_handle_proxy_chat` calls `stats.record_request()` after model is resolved
- [ ] `--stats-db PATH` CLI flag added to tracker
- [ ] 6 unit tests pass; `python -m pytest` passes
