Create the parent's own tracks on a child that already exists.

A clinic-claimed child skips the child form, so this is the same seeding POST /children does, for the one journey that never reaches it. RLS (tracks_family_insert) makes the parent the author and a family member the only caller. A name the parent already has a live track for is skipped, so a retry whose answer was lost never makes a second one. Only the parent's OWN tracks count: a clinician's "Sleep" on the same child is the track this step exists to give the parent an alternative to, so it never cancels their pick.