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

templates/webhook_stats.html

@@ -22,7 +22,11 @@
         </div>
         <div>
             <span class="text-gray-500">Last event</span>
-            <span class="font-medium text-gray-900">{{with .LastEventAt}}{{.UTC.Format "2006-01-02 15:04:05 UTC"}}{{else}}none{{end}}</span>
+            {{if .LastEvent}}
+            <span class="font-medium text-gray-900" title="{{.LastEventUTC}}">{{.LastEvent}}</span>
+            {{else}}
+            <span class="font-medium text-gray-900">none</span>
+            {{end}}
         </div>
         <div>
             <span class="text-gray-500">Retention</span>