Lengthen the WHOIS fetcher's wait after a skipped lookup (closes #222)

A lookup whose RIR is paused is skipped. The skip left the fetcher's wait
as it was. Each lookup asks IANA before it finds its RIR paused. So while
most stale ASes belonged to a paused RIR, the fetcher could ask IANA every
second.

Like a failure, a skipped lookup now makes the wait 1.5 times as long, up
to one minute. The skip is still not counted as a failure, so the status
page's failure figures leave it out.

Model: opus-5-5

internal/routewatch/asnfetcher_internal_test.go

@@ -196,6 +196,48 @@ func TestFetchAndUpdateWaitsOutAPause(t *testing.T) {
 	}
 }
 
+// TestFetchAndUpdateBacksOffDuringAPause looks up an AS at RIPE while RIPE
+// refuses the query. The refusal pauses RIPE for an hour, so every later
+// lookup in the test is skipped. The test sets the WHOIS client's clock and
+// looks the AS up again each time the interval has passed. Like a failure,
+// each skip must make the interval 1.5 times as long, up to one minute. The
+// skips must not count as failures.
+func TestFetchAndUpdateBacksOffDuringAPause(t *testing.T) {
+	t.Parallel()
+
+	f, _ := newFetcherWithWHOISAnswers(t, map[string]string{
+		ianaWHOISServer: ianaRefersToRIPE,
+		ripeWHOISServer: ripeAccessDenied,
+	})
+
+	now := time.Date(2026, 11, 2, 0, 0, 0, 0, time.UTC)
+	f.whoisClient.Now = func() time.Time { return now }
+
+	f.fetchAndUpdate(t.Context(), testASNA)
+
+	for i, want := range []time.Duration{
+		33750 * time.Millisecond,
+		50625 * time.Millisecond,
+		time.Minute,
+		time.Minute,
+	} {
+		now = now.Add(f.getInterval())
+
+		f.fetchAndUpdate(t.Context(), testASNA)
+
+		if got := f.getInterval(); got != want {
+			t.Errorf("interval after %d skipped lookups = %v, want %v",
+				i+1, got, want)
+		}
+	}
+
+	got := f.GetStats()
+	if got.ConsecutiveFails != 1 || got.ErrorsLastHour != 1 {
+		t.Errorf("%d failures in a row and %d errors, want 1 and 1",
+			got.ConsecutiveFails, got.ErrorsLastHour)
+	}
+}
+
 // TestFetchAndUpdateKeepsAskingIANA looks up an AS while IANA's server
 // refuses the connection. The test sets the WHOIS client's clock. The next
 // lookup comes a second before whois.PauseTime has passed. IANA is never