Show each entrypoint's last event and event count on the webhook page (closes #393)

Each entrypoint in the webhook page's entrypoint list now shows when its last event arrived (relative, with the full UTC time on hover), or "never", and how many events arrived through it within the webhook's retention period. Both come from one query per page, grouped by entrypoint, over a new events index on entrypoint_id, deleted_at and created_at, so the page reads only the index entries it counts. Pre-1.0: the index goes into the schema in place. A test checks the database's plan for the statement as the code builds it; another shows two entrypoints with different traffic each with their own figures, an event older than retention left out, and an unused entrypoint reading "never".

Model: opus-5-5

templates/source_detail.html

@@ -97,6 +97,18 @@
                     </div>
                     <!-- The URL above is the entrypoint's credential:
                          anyone holding it can submit events. -->
+                    <div class="text-xs text-gray-500 mt-1">
+                        <span class="font-medium text-gray-700">Last Event:</span>
+                        {{if .LastEvent}}
+                        <span title="{{.LastEventUTC}}">{{.LastEvent}}</span>
+                        {{else}}
+                        <span>never</span>
+                        {{end}}
+                    </div>
+                    <div class="text-xs text-gray-500 mt-1">
+                        <span class="font-medium text-gray-700">Events Within Retention:</span>
+                        <span>{{.Events}}</span>
+                    </div>
                 </div>
                 {{else}}
                 <div class="p-4 text-sm text-gray-500">No entrypoints configured.</div>