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The metrics tracker now also counts the prefixes announced or withdrawn
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 whole number of
+  prefixes announced or withdrawn in the last full minute.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