Cancel Po Group Remainder

Cancel the un-received remainder of an already-raised PO group.

Safety invariant: this endpoint only ever touches quantity that has NEVER been received (no receiving_log entry covers it) — regardless of QC status. It never edits or removes anything with a receiving_log entry, so partially or fully QC'd / inventoried units are structurally impossible to disturb here.

Per real (non-misc) item at po_number, by total_received vs. quantity:

• total_received <= 0 -> revert to qs_state='sel', clear po_number and delivery dates. vendor_opts / selected_vendor / unit_price / quantity are kept intact so the user can re-raise without re-entering anything.
• 0 < total_received < quantity -> shrink this item in place to quantity=total_received, qs_state='purch' (keeps receiving_log/qc_log/po_number untouched). The un-received delta is dropped with no shell row.
• total_received >= quantity -> untouched (already fully received, nothing to cancel)

Misc expense items (shipping/tax — no nickname) ride along with a specific PO and have no purpose once nothing from that PO survives: they are deleted, but ONLY when every real item is deleted too (nothing received anywhere in the group). If any real item keeps a received/kept portion, misc costs legitimately incurred for those goods are left alone.

404 if the PO number has no open (po/purch) items. 400 if every real item is already fully received — there is nothing left to cancel.