List Cancelled Po Groups

Voided POs, rebuilt from state_history — Purchase History's Cancelled filter.

A void deliberately leaves NOTHING to group by: _void_one_item_purchase clears po_number so the line can be re-raised without colliding with the dead order, and resets status to DRAFT so it drops out of _fetch_receiving_items entirely. That is the right shape for the ITEM and the wrong shape for the HISTORY — the order was real, money moved, and the reversal is the record of it.

So the group is reconstructed rather than stored: the void_purchase event carries the PO number it reversed (see _void_one_item_purchase), and the items are gathered back under it here. No column, no migration, and nothing on the live row to keep in sync — the item stays clean and the history is derived.

Read-only and separate from GET /receiving on purpose: this scans DRAFT rows, which the hot receiving path has no reason to pay for on every poll.