fix: Untangle the DB_TYPE env var logic

[berkes]

This logic caused issues when running:

• The defaults in .env.example are wrong.
• The logic sets the DB_TYPE var based on the DB_URL. But this var is not required nor documented. Nor edge-cases like "psql://" etc etc are covered.
• Just enforcing DB_TYPE is much easier.
• We don't support anything else then postgres (and not postgresql, as the error in our typeOrmDateTime says!) so there's no need for logic to support other database types anyway.
• We must set DB_TYPE, because libs like typeOrmDateTime read it from process.env on their own, and not from passed in config. So these libs require this ENV var to be present. So we cannot simply hardcoded it.