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/MaintenanceController.cs

@@ -156,14 +156,7 @@ public class MaintenanceController : Controller
             // Map to DTOs
             var maintenanceDtos = _mapper.Map<List<MaintenanceListDto>>(items);
 
-            // Create paged result
-            var pagedResult = new PagedResult<MaintenanceListDto>
-            {
-                Items = maintenanceDtos,
-                PageNumber = gridRequest.PageNumber,
-                PageSize = gridRequest.PageSize,
-                TotalCount = totalCount
-            };
+            var pagedResult = PagedResult<MaintenanceListDto>.From(gridRequest, maintenanceDtos, totalCount);
 
             // Get equipment name if filtering by equipment
             if (equipmentId.HasValue)