Delete Custom Column

Deletes the column definition AND atomically strips its key out of every affected item's custom_fields (list-scoped column → that list's items; project-wide column → every item across every list in the project) via delete_manual_material_custom_column (migration 20260725000002). Previously this only deleted the row and left the key behind permanently — contradicting the original migration's own comment that claimed this cleanup already happened; this makes that claim true. "Delete for just this list" (as opposed to the whole project) is a client-side choice for project-wide columns — see ManageColumnsModal.js; it hides the column in that list's view instead of calling this endpoint at all, so no partial-delete variant exists here.