Register transactions to track

For transaction.confirmation webhooks, up to 1,000 per call. The transactions your webhooks track at once, across all of them, are capped per plan (Free 500, Hobby 1,000, Starter 10,000, Professional 25,000, Behemoth 100,000; Free's on testnets; each stops counting once its last notify_at level is sent); past that, 403.