Stop a queued or running import (owner/admin)

Stops the transfer for real: the in-flight INSERT … SELECT FROM s3(…) is KILLed on the engine rather than 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 an already-finished import returns 200 with cancelled: false. NOTHING IS ROLLED BACK — an import is not a transaction, so tables it already created still exist and rows it already loaded are still there. The job's final cancelled step names exactly what was created and loaded; TRUNCATE or DROP what you do not want. We deliberately do NOT drop those tables for you: one of them may be a table you already had.