refactor(auth): removed admin role from registration flow.

[CotSezer]

This PR refactors the authentication registration flow to remove assignment of the admin role during user registration. Role elevation is no longer possible at signup.

What changed

• Removed admin role handling from the registration logic
• Cleaned up related conditionals and validations in the auth flow

Why

• Prevents accidental or unauthorized privilege escalation
• Aligns registration with principle of least privilege

Impact / Notes

• No change to existing users or roles
• Admin roles must now be assigned through approved post-registration mechanisms (e.g., internal tooling or migrations)

Testing

• Confirmed admin role cannot be set via registration payload

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Closes #271

[enverern]

It is done. Thank you.