Soft-delete a single marketing preset (admin).

Soft, not hard, for the same reason as the customer path: the row is what a piece of collateral's preset_ids resolves against, and a staff delete would orphan those analyses just as thoroughly. deleted_by records the staff user. Only presets that have never been used to analyze a file can be deleted; a used one returns 409 — see backend/helpers/marketing_presets.py for why, and for what that measurement does not cover. company_id is a match-guard on the lookup (a stale id can't resolve to another company's preset); it is not an access boundary, which is the admin gate above.