inventoryitem: invitQty accepts Infinity/-Infinity — DOUBLE column stores them as inf

[CryptoJones]

Problem

Same class as #171/#179/#193/#205 on InventoryItem. invitQty is typed z.coerce.number(), which accepts the infinities (and "Infinity" from JSON coerces to the float). DOUBLE column stores inf, contaminating PO receiving / inventory-transaction rollups / valuation arithmetic.

Fix

.finite() via shared invitQtyField. Zero + negatives remain valid (out-of-stock, backorders, reconciliation entries).

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