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/ApplicationDbContext.cs

@@ -20,6 +20,7 @@ public class ApplicationDbContext : DbContext
     public DbSet<Job> Jobs => Set<Job>();
     public DbSet<JobPart> JobParts => Set<JobPart>();
     public DbSet<JobStock> JobStocks => Set<JobStock>();
+    public DbSet<PurchaseItem> PurchaseItems => Set<PurchaseItem>();
 
     protected override void OnModelCreating(ModelBuilder modelBuilder)
     {
@@ -281,6 +282,34 @@ public class ApplicationDbContext : DbContext
                 .OnDelete(DeleteBehavior.SetNull);
         });
 
+        // PurchaseItem
+        modelBuilder.Entity<PurchaseItem>(entity =>
+        {
+            entity.HasKey(e => e.Id);
+            entity.Property(e => e.Notes).HasMaxLength(500);
+            entity.Property(e => e.Status)
+                .HasMaxLength(20)
+                .HasConversion(
+                    v => v.ToString(),
+                    v => Enum.Parse<PurchaseItemStatus>(v));
+            entity.Property(e => e.CreatedAt).HasDefaultValueSql("GETUTCDATE()");
+
+            entity.HasOne(e => e.StockItem)
+                .WithMany()
+                .HasForeignKey(e => e.StockItemId)
+                .OnDelete(DeleteBehavior.Cascade);
+
+            entity.HasOne(e => e.Supplier)
+                .WithMany()
+                .HasForeignKey(e => e.SupplierId)
+                .OnDelete(DeleteBehavior.SetNull);
+
+            entity.HasOne(e => e.Job)
+                .WithMany()
+                .HasForeignKey(e => e.JobId)
+                .OnDelete(DeleteBehavior.SetNull);
+        });
+
         // Seed default cutting tools
         modelBuilder.Entity<CuttingTool>().HasData(
             new CuttingTool { Id = 1, Name = "Bandsaw", KerfInches = 0.0625m, IsDefault = true, IsActive = true },

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; }
+}