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

The entrypoint list showed no sign of whether anything uses an entrypoint, so an operator with several could not tell which senders are live before deactivating or deleting one. Each entrypoint now shows when its last event arrived, relative with the UTC time on hover, or "never", and how many events arrived through it within the webhook's retention. The last-event time comes from a new entrypoint_totals row written in the transaction that stores the event and left by retention, so a sender quieter than the retention period does not read "never". The count is one grouped query over a new index. Resubmitted copies count in neither. Pre-1.0: schema changed in place.

Model: opus-5-5

internal/handlers/webhook.go

@@ -326,6 +326,19 @@ func (h *Handlers) createAndFanOut(
 		return nil, nil, err
 	}
 
+	// A resubmitted copy did not arrive on its entrypoint's URL, so it
+	// leaves the entrypoint's last event as it is.
+	if src.ResubmittedFromID == nil {
+		err = database.AddEntrypointTotals(tx, database.EntrypointTotals{
+			EntrypointID: event.EntrypointID, LastEventAt: event.CreatedAt,
+		})
+		if err != nil {
+			tx.Rollback()
+
+			return nil, nil, err
+		}
+	}
+
 	err = tx.Commit().Error
 	if err != nil {
 		return nil, nil, fmt.Errorf(