Backfill deep trade history (and cancel a runaway)

Trades are cursor-paginated and drain the whole window — a day of a liquid pair is millions of rows. Give an explicit start date:

dccd backfill -e okx -s BTC/USDT -t trades --start 2024-01-01

From the web UI, the backfill modal shows live progress and a Stop button (it keeps everything already collected). Via the API:

curl -XDELETE localhost:8080/api/backfill/$RUN_ID