Refactor: Update `auth_user_id` assignment to use LDAP username instead of email

[Miguel7373]

Description of feature or bug
Currently, auth_user_id is assigned by matching email addresses after an OmniAuth login. This is not our long-term strategy. We need to transition to matching based on the Ldap_username or Keycloak_id.

Additionally, we need to address how users are created and synced. Our PTime sync would set auth_user_id, but fails if there is no AuthUser with the Ldap_username or Keycloak_id . Since the Ldap_username gets set on the first Keycloak login (handled in auth_user.rb), we need to shift the auth_user_id assignment logic there.

This ticket also includes a general refactoring of `auth_user.rb to clean up existing technical debt.

Additionally with the next PTime deployment a new Endpoint will be added where we can fetch a user via Ldapname or Keycloak id or Email

Caution

When deploying this a manual PTime sync will probably be mandatory

TODO

• Remove existing logic that sets auth_user_id via email matching.
• Implement logic to set auth_user_id via Ldap_username upon OmniAuth Keycloak login.
• Ensure auth_user_id is successfully set on a user's first login.
• Refactor auth_user.rb.
• Add test if possible

[nevio18324]

I discussed it with @Kagemaru and @Robin481 i think we can already implement this.
The only flaw that exists is that a freshly created PTime user can't use Skills on the first day.
For that a solution would be implementing a PTime endpoint where we can get the PTime user via Ldap username.