Alert on zero-cost PO items when two-way PO sync lands

Right now the inventory sync job filters out PO items with `unitCost: 0` and doesn't email about them because the existing zero-cost data is historical and can't be edited in Zenventory. It'd just spam forever.

But when we add two-way PO sync, we should alert on **new** POs that come in with zero costs — those are actionable and probably mean someone forgot to fill in the cost. The historical ones should stay ignored.

Rough approach: track which PO IDs we've already seen (or compare against last run), and only alert on zero-cost items from POs that are new since the last sync.

Start at the inventory sync job and trace the existing zero-cost filtering and email alert behavior. Inspect how the sync currently identifies purchase orders and persists state, then define how new PO IDs are distinguished from historical ones. Done means new zero-cost PO items alert while previously seen historical items remain ignored.

Written by the indexing model from the issue text.