refactor(env-vars): use envalid in user-mgnt

[naftis]

#5932

Refactors user-mgnt to use envalid and enforce setting environment variables in production. I amended the MINIO_BUCKET to user-mgnt as well, because gateway also has it set explicit in docker-compose.

[github-actions]

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[naftis]

@rikukissa wonder if it should be implicit or explicit in all services? now gateway has it set but user-mgnt didn't. I amended it for consistency.

[rikukissa]

We could make it explicit for all services that use it, yea 👍