Correct the site (or entity-only stamp) of up to 1000 source records and their emission rows

All or nothing: any id that is not a live source record of the organization answers 404 and nothing changes (another organization's record reads as unknown). siteId and entityId cannot both be set; siteId null with no entityId unassigns. Only JSON null unassigns: a blank or whitespace siteId or entityId is a 400. Each changed record gets an audit row. Error codes: 400 VALIDATION_ERROR; 404 NOT_FOUND; 422 SITE_NOT_ASSIGNABLE, ENTITY_NOT_ASSIGNABLE.