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/target_retries_test.go

@@ -383,20 +383,3 @@ func TestTargetRetries_CreateAndEditAgreeOnEveryCase(t *testing.T) {
 		)
 	}
 }
-
-// TestPageOrFirst_CoercesRatherThanRejects pins the one place a
-// non-numeric form value legitimately falls back. A page number says
-// where to send the browser after an action that has already
-// happened, so it is not configuration and rejecting it would report
-// a failure that did not occur.
-func TestPageOrFirst_CoercesRatherThanRejects(t *testing.T) {
-	t.Parallel()
-
-	for _, s := range []string{"", "abc", "0", "-1", "2.7", " "} {
-		assert.Equal(t, 1, handlers.PageOrFirstForTest(s),
-			"%q should fall back to the first page", s)
-	}
-
-	assert.Equal(t, 4, handlers.PageOrFirstForTest("4"))
-	assert.Equal(t, 4, handlers.PageOrFirstForTest(" 4 "))
-}