Event log: only the newest event expanded, and only the 50 most recent (closes #349)

The event log now loads the 50 newest events in its query and opens
only the newest on load, as the recent events on a webhook's page
already do. With 50 at most there is never a second page, so paging is
gone: the page links, the `page` query parameter, the page number that
Replay and Resubmit carried back to the log, and `pageOrFirst` with its
test. The count beside the heading reads "50 most recent of N events"
when there are more. The comments and README lines that named `page` as
the query parameter the service reads now name `next` and `notice`.

Model: opus-5-5

internal/handlers/event_log_view_test.go

@@ -75,9 +75,7 @@ func seedAndProject(
 	wh := seedWebhook(t, db)
 	seedEventWithBody(t, dbMgr, wh.ID, body)
 
-	views := h.LoadEventLogViewsForTest(
-		httptest.NewRecorder(), *wh, 1,
-	)
+	views := h.LoadEventLogViewsForTest(httptest.NewRecorder(), *wh)
 	require.Len(t, views, 1)
 
 	return views[0]