Add a configuration validation system

[mikron-ia]

Currently, if there is a mistake in .env, the administrator is on their own in the matters of debugging. While some mistakes generate obvious errors and crash everything, some of them may generate subtle misbehaviors and sneaky bugs.

Add a console command to validate the configuration step by step - not for the perfection and not too deeply, but enough to issue warnings if something is missing.

Additionally, display a list of missing optional values that use their defaults - this is also a potential bug source.

Prompted by #384 and risks its misconfiguration carries.