Show 50 recent events with size, processing time and status (closes #347)

The recent events list on a webhook's page, now headed "50 Most Recent
Events", shows the 50 newest events, each with its time relative to now
(the full UTC timestamp on hover), its body size, its processing time
and, when the webhook has exactly one HTTP target, that target's last
HTTP status, colour-coded. Resubmitted copies are marked, as in the
event log.

Each event now records its body's size in bytes when it is stored, on
receipt and on resubmit, and the list reads that instead of the body.
Processing time is read from the existing delivery and attempt
timestamps. The list loads its deliveries and their attempts in batched
queries of its own, reading only the columns it shows. A failed load
answers with an error instead of an empty list.

Model: opus-5-5

internal/handlers/handlers.go

@@ -28,7 +28,7 @@ const (
 	// maxBodyShift is the bit shift for 1 MB body limit.
 	maxBodyShift = 20
 	// recentEventLimit is the number of recent events to show.
-	recentEventLimit = 20
+	recentEventLimit = 50
 	// paginationPerPage is the number of items per page.
 	paginationPerPage = 25