Stop a video export that is queued or rendering. The job ends as failed with "Cancelled by user.", anything it charged is refunded, the render stops, and any partial file is deleted. Cancelling a job that already finished or failed changes nothing (cancelled: false), so a repeat is safe. Refuses an unknown job (SIMPLER_STUDIO_EXPORT_JOB_NOT_FOUND): video.export_create returns job ids. Refuses a read-only connection (SIMPLER_MCP_SCOPE_READONLY): connect again with the 'mcp' scope. Free.