`user_created_at` not set for all users

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While checking, if the relevant Nextcloud accounts are correctly disabled based on the policy, I was reading through https://github.com/nextcloud/user_retention/issues/799. While we were aware, that the app can only start counting the days from the day it was installed, I didn't know about the user_created_at user setting mentioned in https://github.com/nextcloud/user_retention/issues/799#issuecomment-2654004910.
Manipulating manually that setting would even allow to make the app work retroactively, even if it was just installed. Great info!

However, while checking the setting of a few accounts, it seems it was not set for all users.
For example, my own account doesn't have that setting set. I assume it is because of it either being a Nextcloud admin or in one of the excluded groups.

While it does make sense to exclude certain accounts from all effects of the app, my account might not be and admin or be part of excluded groups in the future. I expect my account to be treated the same, when that happens. As the setting is not set, this seems to not be the case.

Start with the behavior described in issue #799 and its referenced comment about the user_created_at setting, then trace how this app handles accounts that are admins or belong to excluded groups. Identify the existing entry point and tests for setting or skipping this user setting. Done means eligible users consistently receive the setting and remain correctly handled if their exclusion status changes.