Wire INITIAL_ADMIN_USERNAME into compose once auth service supports it

[matthewhorridge]

Tracking issue for this repo's part of the admin-bootstrap work.

See the main issue in protegeproject/webprotege-authorization-service#36.

What changes here

Once the authorization service supports a config-driven initial admin:

1. Add INITIAL_ADMIN_USERNAME= to .env.example with documentation explaining when to set it.
2. Pass it to the webprotege-authorization-service environment in docker-compose.yml:

   webprotege-authorization-service:
     environment:
       WEBPROTEGE_AUTH_INITIALADMINUSERNAME: ${INITIAL_ADMIN_USERNAME:-}

3. Mention the bootstrap flow in the README (register first user → set INITIAL_ADMIN_USERNAME in .env to their username → restart the auth service → that user is now admin).

When to act

Do nothing until protegeproject/webprotege-authorization-service#36 is implemented and a new version of the auth service image is published.

[matthewhorridge]

See updated findings in protegeproject/webprotege-authorization-service#36 — a bootstrap mechanism already exists via Keycloak client role assignment, so the compose wiring originally proposed may not be needed. The remaining work on this side is likely documentation in the README.