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.Application/Services/CatalogImageService.cs

@@ -47,15 +47,9 @@ public class CatalogImageService : ICatalogImageService
         string? existingImagePath,
         string? existingThumbnailPath)
     {
-        if (file == null || file.Length == 0)
-            return (false, string.Empty, string.Empty, "No file provided.");
-
-        if (file.Length > MaxFileSizeBytes)
-            return (false, string.Empty, string.Empty, "File exceeds the 10 MB limit.");
-
-        var ext = Path.GetExtension(file.FileName).ToLowerInvariant();
-        if (!AllowedExtensions.Contains(ext))
-            return (false, string.Empty, string.Empty, $"File type '{ext}' is not allowed. Accepted types: jpg, jpeg, png, gif, webp.");
+        var (isValid, ext, validationError) = BlobFileHelper.ValidateUpload(file, AllowedExtensions, MaxFileSizeBytes);
+        if (!isValid)
+            return (false, string.Empty, string.Empty, validationError);
 
         var container = _settings.Containers.CatalogImages;
         var blobId = Guid.NewGuid().ToString("N");