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

internal/handlers/export_test.go

@@ -69,6 +69,21 @@ func (s *Handlers) LoadEventLogViewsForTest(
 	return views
 }
 
+// WebhookStatsForTest returns the figures the statistics pane on a
+// webhook's page shows, from the webhook's entrypoints and targets
+// loaded as that page loads them.
+func (s *Handlers) WebhookStatsForTest(webhookID string) *WebhookStats {
+	var entrypoints []database.Entrypoint
+
+	s.db.DB().Where("webhook_id = ?", webhookID).Find(&entrypoints)
+
+	var targets []database.Target
+
+	s.db.DB().Where("webhook_id = ?", webhookID).Find(&targets)
+
+	return s.loadWebhookStats(webhookID, entrypoints, targets)
+}
+
 // AddTemplateForTest registers a template under a page name so that
 // the handlers_test package can drive the render path with a
 // template of its own.