Merge Placeholder

Fold a placeholder user into the one account its id names by SSO identity or email.

The account is added to every team the placeholder is on, then the placeholder is deleted the way DELETE /scim/v2/Users/{id} deletes a user, so the next group push resolves the member value to the real account. Refused with 409 when the row has an SSO identity of its own, owns virtual keys, or names no account or several.