On cloud, GitHub pull request webhooks and Stripe events now run on a
dedicated `webhooks` queue so a busy `high` queue cannot delay them.
Self-hosted instances keep these jobs on `high`.
- Add webhooks_queue() helper and use it in ProcessGithubPullRequestWebhook
and StripeProcessJob
- Provision a `webhooks` Horizon pool in cloud production, sized by
HORIZON_WEBHOOKS_PROCESSES (default 10)
- Document the new pool, rollout and rollback steps
- Cover pool provisioning and job queue routing in tests