Cloudflare deploy: preview deployments for pull requests #467

What

Direct Cloudflare deploys can't publish a pull request's build to its own URL without touching production.

The deploy reference lists no preview option (CLI: deploy).
The only previews documented are the Platform's private previews for connected apps, and those exclude external databases, auth, email and schedules (Connected apps › Private previews).

Today we deploy each pull request with wrangler preview to a Worker Preview named pr-<n>. Each one gets its own URL, and we delete it when the pull request closes.

Done when

CI can deploy a pull request's build to its own URL, apart from production traffic, and remove it when the pull request closes.