NUMTRACKER_AUTH_HOST variable is used for both authn and authz

When starting the server, it is used to specify the authZ host used to check permissions. When running the client, it is the authN provider where user tokens should be sourced.

Running the client from docker compose, eg to configure beamlines for tests, uses the authZ host for authN and falls over.

It is probably sensible to introduce `NUMTRACKER_AUTHN_HOST` and `NUMTRACKER_AUTHZ_HOST` variables to differentiate them. If we can alias the current `NUMTRACKER_AUTH_HOST` variable it would reduce the configuration changes required in deployments.

NB, if/when this is changed, the DLS module file should be updated to use the new variables when running `module load numtracker[/staging]`

Start by reviewing the server and client configuration paths that read NUMTRACKER_AUTH_HOST, the Docker Compose setup, and the DLS module file used by module load numtracker[/staging]. Done means authN and authZ use separate variables, existing deployments can be aliased if supported, and client-based beamline tests no longer use the authZ host.

Written by the indexing model from the issue text.