Add Phase B: Inventory COGS auto-posting to GL on JobUsage transactions

When powder is consumed via a job (JobsController) or scan (InventoryController.LogUsage),
debit the item's CogsAccountId and credit its InventoryAccountId for the cost of the
quantity consumed (using AverageCost if available, else UnitCost). No-op when either
GL account is not configured on the InventoryItem.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

tests/PowderCoating.UnitTests/QuoteAndReworkControllerFlowTests.cs

@@ -138,7 +138,8 @@ public class QuoteAndReworkControllerFlowTests
             Mock.Of<IPricingCalculationService>(),
             new JobItemAssemblyService(),
             Mock.Of<IHubContext<NotificationHub>>(),
-            Mock.Of<IHubContext<ShopHub>>());
+            Mock.Of<IHubContext<ShopHub>>(),
+            Mock.Of<IAccountBalanceService>());
 
         var result = await controller.AddReworkRecord(new CreateReworkRecordDto
         {