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/delivery/export_test.go

@@ -150,6 +150,16 @@ func (e *Engine) ExportDeliverSlack(
 	)
 }
 
+// ExportUpdateDeliveryStatus exposes updateDeliveryStatus. It passes no
+// target type, so no metric moves.
+func (e *Engine) ExportUpdateDeliveryStatus(
+	webhookDB *gorm.DB,
+	d *database.Delivery,
+	status database.DeliveryStatus,
+) error {
+	return e.updateDeliveryStatus(webhookDB, d, "", status)
+}
+
 // ExportProcessNewTask exposes processNewTask.
 func (e *Engine) ExportProcessNewTask(
 	ctx context.Context, task *Task,