Cancel an active signature request and notify its recipients using the same rules as the school interface. Existing signatures and files are preserved. Send an empty object or an optional reason (trimmed, maximum 1000 characters). Successful calls return 200 OK. Repeated calls return the original cancellation, including its reason and timestamp. Completed, refused, expired and provider-managed requests cannot be cancelled (409). Unknown, deleted or foreign documents return 404. API keys require DOCUMENTS write permission; OAuth requires write access. A 503 may occur after persistence: retrying is safe. Emits on_document_cancelled for subscribed integrations.