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>();

CutList.Web/Data/Entities/PurchaseItem.cs

@@ -0,0 +1,25 @@
+namespace CutList.Web.Data.Entities;
+
+public enum PurchaseItemStatus
+{
+    Pending,
+    Ordered,
+    Received
+}
+
+public class PurchaseItem
+{
+    public int Id { get; set; }
+    public int StockItemId { get; set; }
+    public int? SupplierId { get; set; }
+    public int Quantity { get; set; }
+    public int? JobId { get; set; }
+    public string? Notes { get; set; }
+    public PurchaseItemStatus Status { get; set; } = PurchaseItemStatus.Pending;
+    public DateTime CreatedAt { get; set; } = DateTime.UtcNow;
+    public DateTime? UpdatedAt { get; set; }
+
+    public StockItem StockItem { get; set; } = null!;
+    public Supplier? Supplier { get; set; }
+    public Job? Job { get; set; }
+}