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/export_test.go

@@ -51,12 +51,6 @@ const (
 	SidecarLeftMsgForTest = sidecarLeftMsg
 )
 
-// PageOrFirstForTest exposes pageOrFirst for use in the handlers_test
-// package.
-func PageOrFirstForTest(s string) int {
-	return pageOrFirst(s)
-}
-
 // DummyVerificationsForTest reports how many equivalent-cost
 // verifications were charged for usernames that do not exist. It
 // lets a test prove the anti-enumeration path ran without timing
@@ -79,10 +73,9 @@ func TrimPartialRuneForTest(b []byte) []byte {
 func (s *Handlers) LoadEventLogViewsForTest(
 	w http.ResponseWriter,
 	webhook database.Webhook,
-	page int,
 ) []EventLogView {
 	views, _, _ := s.loadEventsWithDeliveries(
-		w, newRequestForTest(), webhook, nil, page,
+		w, newRequestForTest(), webhook, nil,
 	)
 
 	return views