Permanently cancel an internal Telegram campaign

Same gates as campaign creation, plus Idempotency-Key. Atomically compare-and-sets a scheduled, queued, sending or paused campaign to cancelled and stops every queued recipient. Idempotent (changed=false when already cancelled); irreversible — there is no un-cancel through REST. A recipient whose send had already crossed the Telegram submission boundary may still complete; the worker re-checks status immediately before every send. Takes no request body.