Relationship between a sent SMS and tracking records such as notifications and traces.

    This model acts as an extension of a `mail.notification` or a `mailing.trace` and allows to
    update those based on the SMS provider responses both at sending and when later receiving
    sent/delivery reports (see `SmsController`).
    SMS trackers are supposed to be created manually when necessary, and tied to their related
    SMS through the SMS UUID field. (They are not tied to the SMS records directly as those can
    be deleted when sent).

    Note: Only admins/system user should need to access (a fortiori modify) these technical
      records so no "sudo" is used nor should be required here.
    