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/delivery_replay.go

@@ -329,15 +329,25 @@ func replayBody(body string) *string {
 }
 
 // redirectToEventLog redirects a replay or resubmit back to the event
-// log it was triggered from, carrying the outcome as its notice.
+// log it was triggered from, carrying the outcome as its notice. A
+// Replay form carries the list it was pressed in as show, so a replay
+// returns to the Failed or Pending list; a Resubmit form carries none,
+// so a resubmit returns to the full log, where its new event is the
+// newest.
 func redirectToEventLog(
 	w http.ResponseWriter,
 	r *http.Request,
 	webhook database.Webhook,
 	code noticeCode,
 ) {
-	http.Redirect(
-		w, r, withNotice("/hook/"+webhook.ID+"/events", code),
-		http.StatusSeeOther,
+	location := withNotice("/hook/"+webhook.ID+"/events", code)
+
+	show := r.PostFormValue(showParam)
+	if eventLogStatuses(show) != nil {
+		location += "&" + showParam + "=" + show
+	}
+
+	http.Redirect( //nolint:gosec // show is checked by eventLogStatuses
+		w, r, location, http.StatusSeeOther,
 	)
 }