[BUG] User email address uniqueness is only enforced case-sensitive

[SilPan]

Describe the bug
A users email address is checked for uniqueness during creation, but in a case-sensitive way.
Email addresses are case-insensitive tough.

Users with the email addresses e.g.
John.Doe@company.tld
and
john.doe@company.tld
can exist.

Strictly following RFC 5321, the local part of an email address is even case-sensitive.
But in reality everybody assumes, and all major mail providers treat email addresses as case-insensitive.

To Reproduce
Steps to reproduce the behavior:

1. Create new user with email address: John.Doe@company.tld
2. Create new user with email address: john.doe@company.tld
3. Observe in the list of users, that you have two with the same email address.

Expected behavior
Uniqueness of email addresses of users is already checked and a (psycopg2.errors.ForeignKeyViolation) update or delete on table "user" violates foreign key constraint "user_client_user_id_fkey" on table "user_client" is thrown.
This check should be performed case-insensitive.

Desktop (please complete the following information):

• Browser Firefox 140.13.0. ESR
• Version v2.4.27