Creates a document_associations row that links the path asset ID to the supplied document_id. The body is validated with createAssetDocumentSchema; document_id is required and notes is optional. The service inserts entity_type=asset, entity_id from the path, document_id, notes, tenant from context, and created_at, then returns the inserted row. It does not verify that the asset or document belongs to the same tenant before insert, does not set created_by, and publishes no event. Foreign-key or unique constraint errors surface through handleApiError. In the current route wiring, req.context may be absent because no route-level API-key auth wrapper sets it, causing a 500 before insert.