Add a `hidden` boolean to the Biomarker model (migration
0195_biomarker_hidden) so a marker the user no longer needs can be
soft-removed instead of deleted: it leaves the active catalog and the
lab-entry pickers but keeps its readings and canonical unit/range.
Thread `hidden` through the DTO, the update schema, the PUT handler
(field-by-field), and the list endpoint (visible markers first). Regen
the OpenAPI contract.