Refactor: extract shared helpers, fix field drift, add assembly services

- IJobItemAssemblyService / IQuotePricingAssemblyService: centralize job item
  and quote pricing construction that was duplicated across create, rework copy,
  and quote-to-job conversion paths
- BlobFileHelper: single ValidateUpload/GetContentType/SanitizeFileName used by
  6 blob services (JobPhoto, QuotePhoto, ProfilePhoto, CompanyLogo, Equipment,
  Catalog) and BillsController + ExpensesController, removing 8 private copies
- PagedResult<T>.From(): static factory eliminates 6-line boilerplate in 11
  controllers (Appointments, Customers, Equipment, Inventory, Invoices, Jobs,
  Maintenance, CompanyUsers, PlatformUsers, Quotes, Vendors)
- AccountingDropdownHelper: single LoadAsync() call replaces duplicate
  vendor/account/job queries in BillsController and ExpensesController
- JobTemplateItem: add IsSalesItem + Sku fields with migration; propagate
  through JobTemplatesController snapshot copy and GetTemplatesJson projection,
  and JobsController template-application path
- Test assertions updated for standardized BlobFileHelper error messages

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

src/PowderCoating.Web/Controllers/JobTemplatesController.cs

@@ -159,6 +159,8 @@ public class JobTemplatesController : Controller
                 CatalogItemId = item.CatalogItemId,
                 IsGenericItem = item.IsGenericItem,
                 IsLaborItem = item.IsLaborItem,
+                IsSalesItem = item.IsSalesItem,
+                Sku = item.Sku,
                 ManualUnitPrice = item.ManualUnitPrice,
                 RequiresSandblasting = item.RequiresSandblasting,
                 RequiresMasking = item.RequiresMasking,
@@ -248,6 +250,8 @@ public class JobTemplatesController : Controller
                 catalogItemId = i.CatalogItemId,
                 isGenericItem = i.IsGenericItem,
                 isLaborItem = i.IsLaborItem,
+                isSalesItem = i.IsSalesItem,
+                sku = i.Sku,
                 manualUnitPrice = i.ManualUnitPrice,
                 requiresSandblasting = i.RequiresSandblasting,
                 requiresMasking = i.RequiresMasking,