fix: price data update time serialization

price update time was not properly serialized as a number like the schema was, and led to dates being stored as empty objects. UI code also did not check if date was NaN, and rendered "invalid date" instead.

__tests__/priceService.test.ts

@@ -94,7 +94,7 @@ describe('Price Service', () => {
   test('ItemPriceData.toString() returns formatted string', () => {
     const data = new ItemPriceData()
     data.sku = mockSku
-    data.update = mockDate
+    data.update = mockDate.getTime()
     data.ttl = mockTtl
     data.keys = 1
     data.metal = 10.66
@@ -102,6 +102,8 @@ describe('Price Service', () => {
 
     const result = data.toString()
     expect(result).toContain(`Price for ${mockSku}`)
+    expect(result).toContain(mockDate.toString())
+    expect(result).toContain(new Date(mockDate.getTime() + mockTtl).toString())
     expect(result).toContain(`"keys":${data.keys}`)
     expect(result).toContain(`"metal":${data.metal}`)
     expect(result).toContain(`"scmPrice":${data.scmPrice}`)