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 },