fix(horizon): route webhook jobs to a dedicated cloud webhooks queue

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

app/Jobs/ProcessGithubPullRequestWebhook.php

@@ -41,7 +41,7 @@ class ProcessGithubPullRequestWebhook implements ShouldBeEncrypted, ShouldQueue
         public string $fullName,
         public bool $isForkPullRequest = false,
     ) {
-        $this->onQueue('high');
+        $this->onQueue(webhooks_queue());
     }
 
     public function handle(): void