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

Each entrypoint in the webhook page's entrypoint list shows when the last event arrived on its URL (relative, with the full UTC time on hover), or "never" if none ever has, and how many events arrived on it within the webhook's retention period. The last event is kept per entrypoint in a new EntrypointTotals row in the event database, written in the transaction that stores the event, so retention leaves it in place. The count is one query per page, grouped by entrypoint, over a new events index on entrypoint_id, deleted_at, resubmitted_from_id and created_at. Resubmitted copies did not arrive on the URL and count in neither. Pre-1.0: the table and index go into the schema in place.

Model: opus-5-5

internal/database/model_webhook.go

@@ -111,6 +111,13 @@ func (w *Webhook) RetainsForever() bool {
 	return retainsForever(w.RetentionDays)
 }
 
+// RetentionCutoff returns the time before which this webhook's events
+// have expired, as the reaper computes it, and false when the webhook
+// retains them forever.
+func (w *Webhook) RetentionCutoff(now time.Time) (time.Time, bool) {
+	return retentionCutoff(now, w.RetentionDays)
+}
+
 // RetentionLabel returns the webhook's retention policy as display
 // text, so that no template has to know about the sentinel value.
 func (w *Webhook) RetentionLabel() string {