API surface for the 4 newly-migrated tables (PurchaseOrders + InventoryTransactions)

[CryptoJones]

Migration 20260517000000-purchase-orders-and-archive-columns (landed in #39) created four tables that lack API endpoints:

• PurchaseOrderVendors — direct compId scoping
• PurchaseOrderHeaders — direct compId scoping (via povCompId on the vendor it references) OR via own povId
• PurchaseOrderLines — header-scoped
• InventoryTransactions — direct compId scoping

This tracker is for the rollout. Each table gets its own PR following the established CRUD pattern (POST, GET /:id, list-by-parent, PATCH, DELETE; soft-delete via *Arch).

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