List Placeholders

List user rows whose id is another account's SSO identity or email.

An earlier release provisioned a group member it could not match as a user keyed by the raw member value, and that row now shadows the account the value really names, so every push of that member is refused. This lists those rows so an operator can fold each one into the account it shadows with POST /scim/v2/placeholders/{user_id}/merge. A row that has an SSO identity of its own or owns virtual keys is left out: someone uses that account.