Manually correct a document's sensitivity classification

Request body for PATCH /documents/{doc_id}/classification.

Optionally sets the document's effective sensitivity (a RAISE is sticky; a LOWER is content-scoped and additionally requires org:manage — anti-escalation is enforced in the service layer) and/or its audience scope (ENG-648): visibility_mode='audience' + audience_tags narrows the doc to those groups; visibility_mode='org_wide' re-opens it. At least one field must be supplied. reason is recorded on the audit event(s).

Current classification of a document.

Built explicitly (not from_attributes) because document_id maps to Document.id and detected_sensitivity comes from the active version.