Show the last event as a relative time with a UTC tooltip (closes #567)

The Statistics block's "Last event" figure now shows how long ago the
newest event arrived, as its two largest units ("59s", "1m1s", "3d4h"),
worked out when the page is served. Its tooltip is the full time in
UTC. With no event it still shows "none", without a tooltip.

Model: opus-5-5

internal/handlers/webhook_stats_test.go

@@ -265,6 +265,13 @@ func statsPane(t *testing.T, page string) string {
 	return strings.Join(strings.Fields(pane), " ")
 }
 
+// statsLastEventShown matches the pane's last event for an event that
+// arrived at at: a figure with the full time in UTC as its tooltip.
+func statsLastEventShown(at time.Time) string {
+	return `Last event</span>\s*<span [^>]*title="` +
+		at.UTC().Format(time.DateTime) + ` UTC">[^<]+</span>`
+}
+
 // assertStatsTargets checks, for the history seedStatsHistory builds,
 // each target's totals and its deliveries finished in the last 24
 // hours. The first target has ten deliveries and the replay, the
@@ -300,14 +307,17 @@ func assertStatsPaneAfterPrune(
 ) {
 	t.Helper()
 
-	pane := statsPane(t, renderSourceDetailPage(t, h, sess, hist.webhook.ID))
-	lastEvent := hist.newest.CreatedAt.UTC().Format("2006-01-02 15:04:05 UTC")
+	page := renderSourceDetailPage(t, h, sess, hist.webhook.ID)
+	pane := statsPane(t, page)
 
-	assert.Contains(t, pane, "Entrypoints 12 (11 active) "+
-		"Targets 6 (2 active) "+
-		"Deliveries in progress 13 "+
-		"Last event "+lastEvent+" "+
-		"Retention 14 days")
+	// The newest event arrived moments ago, so the last event is a
+	// few seconds ago.
+	assert.Regexp(t, `Entrypoints 12 \(11 active\) `+
+		`Targets 6 \(2 active\) `+
+		`Deliveries in progress 13 `+
+		`Last event \d+s `+
+		`Retention 14 days`, pane)
+	assert.Regexp(t, statsLastEventShown(hist.newest.CreatedAt), page)
 	assert.Contains(t, pane, "Lifetime Within retention "+
 		"Events 10 9 "+
 		"Deliveries 21 18 "+
@@ -440,15 +450,14 @@ func TestWebhookStats_LastEventSurvivesPruningEveryEvent(t *testing.T) {
 	require.NotNil(t, stats.LastEventAt)
 	assert.True(t, arrived.Equal(*stats.LastEventAt))
 
-	pane := statsPane(t, renderSourceDetailPage(t, h, sess, wh.ID))
-	assert.Contains(t, pane,
-		"Last event "+arrived.UTC().Format("2006-01-02 15:04:05 UTC"))
+	assert.Regexp(t, statsLastEventShown(arrived),
+		renderSourceDetailPage(t, h, sess, wh.ID))
 }
 
-// TestWebhookStats_LastEventInUTC checks that the pane shows when the
-// last event arrived in UTC, as the event list does, when the time was
-// stored in another zone, as it is on a host whose local time is not
-// UTC.
+// TestWebhookStats_LastEventInUTC checks that the tooltip of the pane's
+// last event gives the time in UTC, as the event list does, when the
+// time was stored in another zone, as it is on a host whose local time
+// is not UTC.
 func TestWebhookStats_LastEventInUTC(t *testing.T) {
 	t.Parallel()
 
@@ -474,8 +483,9 @@ func TestWebhookStats_LastEventInUTC(t *testing.T) {
 	require.NoError(t, database.AddEventTotals(webhookDB,
 		database.EventTotals{Events: 1, LastEventAt: &arrived}))
 
-	pane := statsPane(t, renderSourceDetailPage(t, h, sess, wh.ID))
-	assert.Contains(t, pane, "Last event 2026-03-05 03:30:00 UTC")
+	assert.Regexp(t,
+		`Last event</span>\s*<span [^>]*title="2026-03-05 03:30:00 UTC">`,
+		renderSourceDetailPage(t, h, sess, wh.ID))
 }
 
 // TestWebhookStats_PaneShowsRetentionPeriod checks that the statistics
@@ -541,8 +551,11 @@ func TestWebhookStats_WebhookWithNoEvents(t *testing.T) {
 	assert.Equal(t, &handlers.WebhookStats{}, h.WebhookStatsForTest(wh.ID))
 	assert.Equal(t, "—", handlers.RecentWindow{}.FailurePercent())
 
-	pane := statsPane(t, renderSourceDetailPage(t, h, sess, wh.ID))
+	page := renderSourceDetailPage(t, h, sess, wh.ID)
+	pane := statsPane(t, page)
 	assert.Contains(t, pane, "Last event none")
+	assert.Regexp(t, `Last event</span>\s*<span class="[^"]*">none</span>`,
+		page, "a last event of none has a tooltip")
 	assert.Contains(t, pane, "Failure percentage — —")
 	assert.False(t, dbMgr.DBExists(wh.ID))
 }
@@ -567,3 +580,35 @@ func TestRecentWindow_FailurePercent(t *testing.T) {
 		assert.Equal(t, tt.want, tt.window.FailurePercent(), tt.window)
 	}
 }
+
+// TestRelativeTime pins how the pane writes the time since the last
+// event, at each boundary between its units.
+func TestRelativeTime(t *testing.T) {
+	t.Parallel()
+
+	now := time.Date(2026, time.October, 26, 12, 0, 0, 0, time.UTC)
+
+	tests := []struct {
+		ago  time.Duration
+		want string
+	}{
+		{0, "0s"},
+		{59900 * time.Millisecond, "59s"},
+		{60 * time.Second, "1m"},
+		{61 * time.Second, "1m1s"},
+		{3599 * time.Second, "59m59s"},
+		{3600 * time.Second, "1h"},
+		{3660 * time.Second, "1h1m"},
+		{86399 * time.Second, "23h59m"},
+		{86400 * time.Second, "1d"},
+		{90000 * time.Second, "1d1h"},
+		{400*24*time.Hour + 3*time.Hour, "400d3h"},
+		// An event time ahead of the server clock.
+		{-time.Hour, "0s"},
+	}
+
+	for _, tt := range tests {
+		assert.Equal(t, tt.want,
+			handlers.RelativeTimeForTest(now.Add(-tt.ago), now), tt.ago)
+	}
+}