Validate Backend config at startup

It might be helpful to define some kind of schema and/or rules for the structure and content of the Backend config.

The schema/rules could then be checked at startup, and fail fast if there are issues.

Some examples of possible rules:

* `default.database` should be an object
* `default.server.port` should be an integer
* `default.env.sysadminAccount` should be an email
* `default.oidc.enabled` should be a `boolean` if it is present
* `default.oidc.issuerUrl` should be a `url` if it is present

etc.