Rewrite a column's card order in one transaction

Renumbers the listed cards onto column with positions 1, 2, 3, … in request order. Sized for a real column (up to 10 000 ids), not an import batch - the board uses this when a drop can no longer find a representable midpoint between two neighbours. All ids must already belong to this board (404 otherwise). Within-board column changes are recorded to each card's activity log. All-or- nothing; no notification emails. Body limit 1 MiB.