Show each target's delivered and failed deliveries in the target list (closes #372)

Each row of the webhook page's target list now shows the target's
delivered and failed deliveries, in total and in the last 24 hours.
The totals are the target's running totals, so retention does not
reduce them; the 24-hour figures are counted over that window of the
deliveries' status index, in one query for all targets. Deliveries
still pending or retrying count in neither.

Model: opus-5-5

templates/source_detail.html

@@ -259,6 +259,18 @@
                     </div>
                     {{end}}
                     {{end}}
+                    {{with .Deliveries}}
+                    <div class="text-xs text-gray-500 mt-1">
+                        <span class="font-medium text-gray-700">Delivered:</span>
+                        <span>{{.Delivered}} in total, {{.DeliveredLast24Hours}} in the last 24 hours</span>
+                    </div>
+                    <div class="text-xs text-gray-500 mt-1">
+                        <span class="font-medium text-gray-700">Failed:</span>
+                        <span>{{.Failed}} in total, {{.FailedLast24Hours}} in the last 24 hours</span>
+                    </div>
+                    {{else}}
+                    <div class="text-xs text-gray-500 mt-1">The delivery counts could not be read.</div>
+                    {{end}}
                 </div>
                 {{else}}
                 <div class="p-4 text-sm text-gray-500">No targets configured.</div>