Get the exact JSON a webhook delivery POSTed

Returns the exact JSON body POSTed to the webhook endpoint for a WEBHOOK-channel delivery, reconstructed on demand (never stored) so it matches the delivered bytes. Member-only — the payload carries every match's full content, so unlike the delivery view (which URL possession alone can read), this requires membership of the owning organization. Returns 404 for a non-webhook delivery and 403 for a non-member. Split from the delivery view because for a large digest this reconstructs to many MB; fetch it on demand only when has_webhook_payload is true.