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The metrics tracker now also counts the prefixes announced or withdrawn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

internal/templates/index.html

@@ -400,7 +400,7 @@
                     }
 
                     const totalUpdates = data.ipv4_updates_per_sec + data.ipv6_updates_per_sec;
-                    document.getElementById('stat-updates').textContent = totalUpdates.toFixed(1);
+                    document.getElementById('stat-updates').textContent = Math.round(totalUpdates).toLocaleString();
                 })
                 .catch(() => {
                     document.getElementById('stat-status').textContent = 'Error';