Check Po Number Exists

Check whether a PO number is already in use.

Two scopes, depending on whether project_id is passed:

• Given: unchanged from before — exact-project check, used by the receipt-scan flow (AddItemPanel.js), where the PO number is user-typed and a typo colliding with an existing unrelated PO would silently merge two different purchases. That caller treats this as a warning, not a hard block — merging into an existing PO can be intentional there (e.g. scanning a second receipt for the same order).
• Omitted: checks across every project in the CALLER'S OWN SHOP plus the shop's own facility list (INY- restock POs live there with project_id NULL), not the whole multi-tenant database (see _caller_shop_filter — same convention the cross-project /receiving* dashboards use). This is what RaisePOConfirmModal's custom PO# input and the post-raise rename field use — Vinay's shop runs its own numbering scheme, which (unlike our auto-generated <project_number>-P### format) isn't guaranteed unique per-project, so those callers treat a collision as a hard block, not just a warning.