AddPantryItem

Add + partial-update request body for /v1/pantry/items[{id}]. `ingredient_id` is required on POST, omitted on PATCH (updates cannot move an item to a different ingredient — that would invalidate the unique (household_id, ingredient_id) constraint without an explicit delete + add). Every other field is `Optional`: omitted means "leave unchanged on PATCH" and "use the column default on POST". Caps: - MAX_QUANTITY_AMOUNT bounds the decimal so the worker cannot overflow the decimal(10,3) column from a hostile payload. - MAX_QUANTITY_UNIT_LENGTH matches the storage column width.