Event log: show only the events with a failed or a pending delivery (closes #390)

The event log gains three links above the list: All, Failed (N) and Pending (N), carried in a `show` query parameter (`failed`, `pending`); any other value shows every event. Failed lists the events with at least one failed delivery, Pending those with one pending or retrying, under the same 50-row limit and newest-first order as the full list. Events are picked by their deliveries' status through `idx_deliveries_status`; each count is the number of distinct matching events, read from the deliveries alone, and can exceed the 50 shown. An empty filtered list says that no event matches. The README and the comments that name the query parameters the service reads now include `show`.

Model: opus-5-5

internal/handlers/export_test.go

@@ -75,7 +75,7 @@ func (s *Handlers) LoadEventLogViewsForTest(
 	webhook database.Webhook,
 ) []EventLogView {
 	views, _, _ := s.loadEventsWithDeliveries(
-		w, newRequestForTest(), webhook, nil,
+		w, newRequestForTest(), webhook, nil, nil,
 	)
 
 	return views