Offer no Replay for a delivery to a deleted target (closes #387)

The event log offered Replay on a delivery whose target was deleted,
and the refusal told the operator to recreate the target and replay.
That cannot work: a recreated target is a new one the old delivery
does not name, so the replay is refused again. The Replay button is
now left off such a delivery, whose row already names the target as
deleted, and the refusal a page left open can still reach now points
to Resubmit, which sends the event to the webhook's currently active
targets.

Model: opus-5-5

internal/handlers/delivery_replay.go

@@ -21,7 +21,9 @@ const (
 	// replayTargetDeleted reports a target that once existed and has
 	// since been deleted. Deletes are soft and deliveries carry no
 	// foreign key to the target row, so the history survives its
-	// target and this is the ordinary case for an old event.
+	// target and this is the ordinary case for an old event. The
+	// event log shows no Replay button for such a delivery, so only
+	// a page loaded before the delete reaches this.
 	replayTargetDeleted noticeCode = "replay-target-deleted"
 
 	// replayTargetMissing reports a target id that names no row at