feat: auto-create inventory records for new products + migration worker

- CreateNewProductsCommandHandler: inject IInventoryService, auto-init
  inventory record with qty=0 when a new regular product is created
  (DiscountProduct handler already had this)
- InventoryInitializerService: one-time BackgroundService that runs on
  startup, finds existing products without InventoryItem records, and
  creates them (migration for legacy products)
- Register InventoryInitializerService in DI

src/CMSMicroservice.Infrastructure/ConfigureServices.cs

@@ -123,6 +123,9 @@ public static class ConfigureServices
         // Expire pending discount orders after 30 minutes
         services.AddHostedService<ExpirePendingOrdersService>();
         
+        // One-time: Initialize inventory records for existing products
+        services.AddHostedService<InventoryInitializerService>();
+        
         if (configuration.GetValue<bool>("UseInMemoryDatabase"))
         {
             services.AddDbContext<ApplicationDbContext>(options =>