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.Infrastructure/Migrations/ApplicationDbContextModelSnapshot.cs

@@ -4833,6 +4833,9 @@ namespace PowderCoating.Infrastructure.Migrations
                     b.Property<bool>("IsLaborItem")
                         .HasColumnType("bit");
 
+                    b.Property<bool>("IsSalesItem")
+                        .HasColumnType("bit");
+
                     b.Property<int>("JobTemplateId")
                         .HasColumnType("int");
 
@@ -4851,6 +4854,9 @@ namespace PowderCoating.Infrastructure.Migrations
                     b.Property<bool>("RequiresSandblasting")
                         .HasColumnType("bit");
 
+                    b.Property<string>("Sku")
+                        .HasColumnType("nvarchar(max)");
+
                     b.Property<decimal>("SurfaceAreaSqFt")
                         .HasColumnType("decimal(18,2)");
 
@@ -6071,7 +6077,7 @@ namespace PowderCoating.Infrastructure.Migrations
                         {
                             Id = 1,
                             CompanyId = 0,
-                            CreatedAt = new DateTime(2026, 5, 8, 14, 21, 51, 589, DateTimeKind.Utc).AddTicks(4358),
+                            CreatedAt = new DateTime(2026, 5, 10, 1, 12, 48, 386, DateTimeKind.Utc).AddTicks(2249),
                             Description = "Standard pricing for regular customers",
                             DiscountPercent = 0m,
                             IsActive = true,
@@ -6082,7 +6088,7 @@ namespace PowderCoating.Infrastructure.Migrations
                         {
                             Id = 2,
                             CompanyId = 0,
-                            CreatedAt = new DateTime(2026, 5, 8, 14, 21, 51, 589, DateTimeKind.Utc).AddTicks(4424),
+                            CreatedAt = new DateTime(2026, 5, 10, 1, 12, 48, 386, DateTimeKind.Utc).AddTicks(2260),
                             Description = "5% discount for preferred customers",
                             DiscountPercent = 5m,
                             IsActive = true,
@@ -6093,7 +6099,7 @@ namespace PowderCoating.Infrastructure.Migrations
                         {
                             Id = 3,
                             CompanyId = 0,
-                            CreatedAt = new DateTime(2026, 5, 8, 14, 21, 51, 589, DateTimeKind.Utc).AddTicks(4426),
+                            CreatedAt = new DateTime(2026, 5, 10, 1, 12, 48, 386, DateTimeKind.Utc).AddTicks(2261),
                             Description = "10% discount for premium customers",
                             DiscountPercent = 10m,
                             IsActive = true,