Discard Batch Endpoint

Delete every member row of batch_key (collection parquet untouched).

Takes the same transaction-scoped advisory lock as :force-merge and the automatic seal, keyed on the collection id, BEFORE deleting anything — a concurrent seal that already loaded these members (pre-lock) would otherwise re-upload them into the collection right after we discard them, orphaning the write. If the discarded batch is the currently-sealed one, the folder's sealed_batch_key done-marker is cleared in the same transaction so a later delivery resolving to this key isn't silently skipped as "already sealed".