Add a statistics pane to the webhook page (closes #368)

Each webhook's event database keeps one row of running totals: events,
deliveries and failures, and how many of each retention removed.
Storing an event, creating a delivery, a delivery becoming failed and
the retention sweep each update it in the transaction that writes or
deletes the rows it counts. Deliveries get a finished_at column, the
last column of the status index, so the last-10-minutes and
last-24-hours figures are index-range counts. The pane is its own
template, included at the top of the page.

The schema changes in place with nothing back-filled, so an existing
database must be recreated.

Model: opus-5-5

templates/source_detail.html

@@ -24,6 +24,8 @@
         </div>
     </div>
 
+    {{template "webhook_stats" .}}
+
     <div class="grid grid-cols-1 lg:grid-cols-2 gap-6">
         <!-- Entrypoints -->
         <div class="card">