auth Admin SDK now initialises inside a Nitro startup plugin

The auth Admin SDK was being initialised lazily on first auth call. On the very first cold request after a deploy, the lazy init introduced a noticeable extra latency before the first auth check resolved.

The Admin SDK now initialises inside a dedicated Nitro startup plugin, so by the time the first request lands, the SDK is already warm. The change shaved a meaningful tail off the cold-start latency on both the admin app and the feedback portal.