Get Next Po Number

Return the next sequential PO number, minted from the right counter.

Scope decides which counter, because Jeevy has two and they must not be crossed: a project's <project_number>-P<seq> and the shop's INY-YYMMDD-P<seq> for facility stock.

• project_id given -> next_vendor_po_number(project_id)
• project_id absent -> next_shop_inventory_po_number(session shop)

project_id became optional when the sign-off gate started routing facility restocks to qs_state='approval'. Approving one has to mint a number, and there is no project to mint from — the approve path used to hard-fail with "no project context for this item", stranding the order permanently.

The shop comes from the SESSION, never a parameter: it is an identity, and accepting it from the caller would let one shop spend another's sequence.

Both RPCs atomically increment and never reuse, so consuming this endpoint is intentional — the caller is expected to store what it gets back.