docs(dashboard): add /api/stats-hive to the endpoint reference

docs/web-ui/dashboard.md

@@ -832,6 +832,12 @@ that's a browser-level decision, not ours.
   `/dashboard/history` backfill.
 - `GET /api/reminders` — list pending reminders for the
   dashboard's queued-reminders panel.
+- `GET /api/stats-hive?window=<1h|4h|24h|3d|7d|30d>` — hive-wide
+  turn-stats rollup for the ST4TS tab. Aggregates every agent's
+  `hyperhive-turn-stats.sqlite` read-only (skips missing / unreadable /
+  zero-turn dbs); returns swarm totals, a busiest-first per-agent
+  rollup, swarm model mix, and a labelled `est_cost_usd`. Window
+  defaults to `24h`.
 - `POST /cancel-reminder/{id}` — hard-delete a pending reminder.
 - `POST /retry-reminder/{id}` — re-arm a reminder whose delivery
   failed (clears the failure state so the scheduler retries).