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

internal/metrics/minutecounter_internal_test.go

@@ -0,0 +1,86 @@
+package metrics
+
+import (
+	"sync"
+	"testing"
+	"time"
+)
+
+// TestChurnPerMinuteCountsLastFullMinute announces and withdraws prefixes on
+// an injected clock and expects each minute's whole count once that minute
+// has ended, and 0 for a minute with none.
+func TestChurnPerMinuteCountsLastFullMinute(t *testing.T) {
+	t.Parallel()
+
+	now := time.Date(2026, 10, 26, 12, 0, 30, 0, time.UTC)
+	tracker := New()
+	tracker.churnPerMinute.now = func() time.Time { return now }
+
+	expectChurn := func(want int, why string) {
+		t.Helper()
+
+		got := tracker.GetChurnPerMinute()
+		if got != want {
+			t.Fatalf("at %s churn = %d, want %d: %s",
+				now.Format(time.TimeOnly), got, want, why)
+		}
+	}
+
+	const firstMinuteChurn = 2
+
+	tracker.RecordAnnouncement()
+	tracker.RecordWithdrawal()
+	expectChurn(0, "12:00 has not ended")
+
+	now = time.Date(2026, 10, 26, 12, 1, 59, 0, time.UTC)
+
+	tracker.RecordAnnouncement()
+	expectChurn(firstMinuteChurn, "12:00 had one announcement and one withdrawal")
+
+	now = time.Date(2026, 10, 26, 12, 2, 0, 0, time.UTC)
+
+	expectChurn(1, "12:01 had one announcement")
+
+	now = time.Date(2026, 10, 26, 12, 4, 0, 0, time.UTC)
+
+	expectChurn(0, "12:03 had none")
+
+	tracker.RecordWithdrawal()
+
+	now = time.Date(2026, 10, 26, 12, 6, 0, 0, time.UTC)
+
+	expectChurn(0, "12:05 had none, though 12:04 had one withdrawal")
+}
+
+// TestChurnPerMinuteCountsConcurrentRecords announces prefixes from several
+// goroutines at once and expects every one counted.
+func TestChurnPerMinuteCountsConcurrentRecords(t *testing.T) {
+	t.Parallel()
+
+	const (
+		goroutines          = 8
+		recordsPerGoroutine = 1000
+	)
+
+	now := time.Date(2026, 10, 26, 12, 0, 0, 0, time.UTC)
+	tracker := New()
+	tracker.churnPerMinute.now = func() time.Time { return now }
+
+	var wg sync.WaitGroup
+	for range goroutines {
+		wg.Go(func() {
+			for range recordsPerGoroutine {
+				tracker.RecordAnnouncement()
+			}
+		})
+	}
+
+	wg.Wait()
+
+	now = now.Add(time.Minute)
+
+	want := goroutines * recordsPerGoroutine
+	if got := tracker.GetChurnPerMinute(); got != want {
+		t.Fatalf("churn = %d, want %d", got, want)
+	}
+}