Show a database target's archive file, size and last write in the target list (closes #397)

Each database target's row on the webhook page names its archive
file, as delivery.ArchivePath finds it, and shows its size on disk and
when it was last written, or "not created yet" when there is no file.
The figures come from the metadata of the file and its -wal only: the
size is the two together, and the last write the later of their
modification times, since a write lands in the -wal first.

Model: opus-5-5

internal/handlers/source_management.go

@@ -521,7 +521,7 @@ func (h *Handlers) renderSourceDetail(
 		// target's stored config blob holds a credential, and it
 		// must never reach a template.
 		"Entrypoints": NewEntrypointViews(entrypoints),
-		"Targets":     delivery.NewTargetViews(targets),
+		"Targets":     h.targetRows(&webhook, targets),
 		"Events":      events,
 		"BaseURL":     baseURL,
 		"Stats":       h.loadWebhookStats(webhook.ID, entrypoints, targets),