Update healthcheck for local playground

[mkurapov]

Context

Now that we added required authentication to the Admin API, our backend healthcheck when starting up the local environment is failing (since the request returns 401).

What we should do is add a /healthz route to the backend Admin API (similar to the Open Payments API) before the GraphQL resolvers are added and use that instead in the docker compose files for the local environment.

Is anyone else experiencing an issue where the local environment's backend service fails the health check? I'm getting it when I try to spin it up on main, even.

Slack Message

Todo

• Add healthz route to Admin API before the resolvers are registered
• Update healthcheck for docker compose files cloud-nine and happy-life backends to use healthz Admin API route