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

Each entrypoint in the webhook page's entrypoint list now shows when its last event arrived (relative, with the full UTC time on hover), or "never", and how many events arrived through it within the webhook's retention period. Both come from one query per page, grouped by entrypoint, over a new events index on entrypoint_id, deleted_at and created_at, so the page reads only the index entries it counts. Pre-1.0: the index goes into the schema in place. A test checks the database's plan for the statement as the code builds it; another shows two entrypoints with different traffic each with their own figures, an event older than retention left out, and an unused entrypoint reading "never".

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 {