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; any other value shows
every event. Failed lists the events with a failed delivery, Pending
those with one pending or retrying, each once, under the full list's
50-row limit and order. A filtered list finds the matching deliveries
through `idx_deliveries_status`, looks up their events by ID and sorts
them, and reads the rows of only the events shown; its counts read the
deliveries alone. The line beside the heading says what a filter
counts, and Replay returns to the list it was pressed in. The README
and the comments naming the query parameters the service reads 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