- normalize username into lowercase (this is already ASCII only)
- permit users to login with their username with insensitive case
- if a disabled account is reactivated and got a collision, it will normalize it too
- server will stop on collision (ex: 2 entries with `Alice` and `alice`)
in a such case admin can run:
```
oxicloud migrate lowercase-usernames --dry-run
```
then
```
oxicloud migrate lowercase-usernames
```