bug: can create invite for email that already has an active account

[ayeo]

Description

It's possible to create an invitation for an email address that already has an active account in the system. The invite is created successfully, but when the user tries to accept it via the new-user flow, it fails with "Account already exists."

Steps to reproduce

1. Register a user with admin@test.com
2. Create an invite for admin@test.com to the same or different org
3. Invite is created successfully — should warn or redirect to existing-user flow

Expected behavior

The new-user accept endpoint should handle this gracefully — either reject the invite creation, or redirect to the existing-user accept flow instead of failing with a confusing error.