Show route churn as a whole count per minute (closes #160)

The status page showed route churn per second as a one-minute moving
average, so a quiet feed gave a fraction such as 0.1. The average was
correct.

The metrics tracker now also counts prefix announcements and
withdrawals, from all peers, in each clock minute. /api/v1/stats
reports the count for the last full minute as route_churn_per_min in
stream, in place of route_churn_per_sec. The status page shows it as
"Route Churn/min".

The status and home pages round messages and updates per second to
whole numbers.

Model: opus-5-5

TODO.md

@@ -33,6 +33,11 @@ tested.
 
 # Completed Steps
 
+- 2026-10-26: the status page shows "Route Churn/min", the number of prefix
+  announcements and withdrawals, from all peers, in the last full clock minute.
+  `/api/v1/stats` reports it as `route_churn_per_min` in `stream`, which
+  replaces the moving average `route_churn_per_sec`. The messages and updates
+  per second on `/` and `/status` are whole numbers (closes #160)
 - 2026-10-26: the WHOIS fetcher backs off more slowly. A failure makes its wait
   1.5 times as long, up to one minute instead of five. Its queries to RIPE,
   APNIC and AFRINIC leave out person and role objects, which RIPE counts against