Backfill Railway Webhooks

Register ready apps' webhooks, up to limit per call.

Default: only apps with no webhook yet — call again for the next batch.

include_registered=true is the repair after RAILWAY_WEBHOOK_SECRET or APP_MANAGER_PUBLIC_URL changes. Every existing registration then points at a URL that 404s, and nothing else re-registers an app that already has a webhook id. It walks every ready app, replacing stale registrations and adopting current ones; page through with offset.

Sequential and capped, because it runs inside a request: ~3 Railway calls per app, and a whole fleet in one call would outlast the proxy's timeout.