Copy a public list into the caller's own saved lists ("Save list").

The members are CLONED row-for-row rather than re-saved through /add-place: that path keys on a canonical (or Google) place id, and an imported list's places routinely have place_id IS NULL — for those the denormalized row is the place, so re-saving would drop them. The copy is a snapshot: the new owner can rename/delete it, and later edits by the original owner don't propagate.

Idempotent for social lists via the same (user_id, source, source_post_code) key the re-import dedup uses — saving a reel-derived list twice, or saving one you already imported yourself, returns the list you already have instead of a duplicate.