Stop a queued or running export (owner/admin)

Stops the transfer for real: the in-flight INSERT INTO FUNCTION s3(…) is KILLed on the engine, not merely marked cancelled, so it stops consuming resources immediately — killed_statements reports how many were actually stopped (0 is normal if the job was between statements). IDEMPOTENT: cancelling a job that has already finished returns 200 with cancelled: false rather than an error. An export only READS your data, so cancelling one cannot have changed your project; any partial objects already written are never downloadable (only a succeeded export produces links) and expire on the bucket's own lifecycle.