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/Data/Entities/Job.cs

@@ -10,6 +10,9 @@ public class Job
     public string? Notes { get; set; }
     public DateTime CreatedAt { get; set; } = DateTime.UtcNow;
     public DateTime? UpdatedAt { get; set; }
+    public DateTime? LockedAt { get; set; }
+
+    public bool IsLocked => LockedAt.HasValue;
 
     public CuttingTool? CuttingTool { get; set; }
     public ICollection<JobPart> Parts { get; set; } = new List<JobPart>();