feat: Add PurchaseItem entity and job locking data layer

Add PurchaseItem entity with status tracking (Pending/Ordered/Received),
supplier and job relationships. Add LockedAt timestamp to Job entity for
controlling editability after materials are ordered. Includes
PurchaseItemService (CRUD + bulk create), JobService Lock/Unlock methods,
EF Core migrations, and DI registration.

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

CutList.Web/Program.cs

@@ -21,6 +21,7 @@ builder.Services.AddScoped<StockItemService>();
 builder.Services.AddScoped<JobService>();
 builder.Services.AddScoped<CutListPackingService>();
 builder.Services.AddScoped<ReportService>();
+builder.Services.AddScoped<PurchaseItemService>();
 
 var app = builder.Build();