Reduce webhook delivery cleanup impact (#2550)

* Add index on webhook_deliveries.created_at

The stale scope (WHERE created_at < ?) and ordered scope
(ORDER BY created_at DESC) both benefit from this index.
Without it, Webhook::Delivery.cleanup does a full table scan.

* Run webhook delivery cleanup every 15 minutes

Each run now deletes ~15 minutes of newly-stale records instead
of ~4 hours worth, a 16x reduction in per-run write volume.

config/recurring.yml

@@ -20,7 +20,7 @@ production: &production
     schedule: every hour at minute 12
   cleanup_webhook_deliveries:
     command: "Webhook::Delivery.cleanup"
-    schedule: every 4 hours at minute 51
+    schedule: every 15 minutes
   cleanup_magic_links:
     command: "MagicLink.cleanup"
     schedule: every 4 hours