Creates a catalog item.

Requires ManageInventory and an Idempotency-Key header. Same as adding an item in the app: the item gets the organization's default "No Category" when categoryId is omitted, its Cost and Unit Price properties are created (0 unless cost / unitPrice are supplied, stored to 2 decimals) and the category's properties are copied onto it. An active item with the same name is refused with 409 (the rule the app's template builder applies). Creates no stock — follow up with POST /api/v1/items/{id}/stock/adjust. No webhook fires for a create. Permissions mirror the app exactly: the Items page creates items, sets price/cost and category under ManageInventory, so that is all this endpoint needs — except description, which in the app is edited only on the Item Templates page and so additionally requires ManageTemplates (403 otherwise; omit the field or grant the permission to the key). Example: { "name": "1/2\" PEX Coupling", "categoryId": 4, "unitPrice": 1.25, "cost": 0.40 }.