Return 409 Conflict for duplicate email registration

Duplicate email registration now returns 409 Conflict with a dedicated AUTH_EMAIL_EXISTS error code. Previously, all registration failures returned 400 Bad Request with AUTH_REGISTRATION_FAILED, forcing callers to pattern-match on the details string to detect duplicates.

Callers can now match on the code field directly:

{
  "code": "AUTH_EMAIL_EXISTS",
  "message": "A user with this email already exists.",
  "details": null,
  "hint": "Try logging in instead."
}

Other registration failures continue to return 400 with AUTH_REGISTRATION_FAILED.

For the full error code reference, see User Tokens — Error Codes.