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/server/sentry.go

@@ -218,8 +218,9 @@ func keptSentryHeaders(headers map[string]string) map[string]string {
 
 // sentryKeepsHeader reports whether a request header is routing or
 // content metadata rather than client-chosen payload. Referer is kept
-// on the reasoning that it is browser-set, that this service emits
-// only ?page= in its own links, and that Referrer-Policy is set to
+// on the reasoning that it is browser-set, that the only query
+// parameters in this service's own URLs are the login page's `next`
+// and `notice`, and that Referrer-Policy is set to
 // strict-origin-when-cross-origin. X-Request-Id ties the event to the
 // local access log line, which holds the rest of the detail.
 func sentryKeepsHeader(name string) bool {