Delete public user accounts

[hartsick]

But keep the admins.

.delete_all intentionally skips activemodel callbacks

I tested this locally on small set of test data. Would appreciate a lookover to make sure:

1. there aren't any other accounts we should exclude from deletion
2. confirm we shouldn't run callbacks on deletion (I skipped them because it felt too risky/destructive)
3. any other gotchas I'm missing

Task can be ran as bin/rake users:delete_nonadmins

[esoterik]

The analytics page in the admin still loads fine after running this and shows data, so I think this is good to merge! The deletion took < 30s on my local db, which is a dump from october 2023 or 2024, so hopefully shouldn't take too long on prod.

[hartsick]

thank you!!