feat: Add StockItem and SupplierOffering entities

Introduce new data model that separates stock catalog (StockItem) from
supplier-specific pricing/catalog info (SupplierOffering). StockItem
represents a Material+Length combination, while SupplierOffering links
suppliers to stock items with part numbers, descriptions, and pricing.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

CutList.Web/Data/Entities/StockItem.cs

@@ -0,0 +1,15 @@
+namespace CutList.Web.Data.Entities;
+
+public class StockItem
+{
+    public int Id { get; set; }
+    public int MaterialId { get; set; }
+    public decimal LengthInches { get; set; }
+    public string? Name { get; set; }
+    public bool IsActive { get; set; } = true;
+    public DateTime CreatedAt { get; set; } = DateTime.UtcNow;
+    public DateTime? UpdatedAt { get; set; }
+
+    public Material Material { get; set; } = null!;
+    public ICollection<SupplierOffering> SupplierOfferings { get; set; } = new List<SupplierOffering>();
+}

CutList.Web/Data/Entities/SupplierOffering.cs

@@ -0,0 +1,16 @@
+namespace CutList.Web.Data.Entities;
+
+public class SupplierOffering
+{
+    public int Id { get; set; }
+    public int StockItemId { get; set; }
+    public int SupplierId { get; set; }
+    public string? PartNumber { get; set; }
+    public string? SupplierDescription { get; set; }
+    public decimal? Price { get; set; }
+    public string? Notes { get; set; }
+    public bool IsActive { get; set; } = true;
+
+    public StockItem StockItem { get; set; } = null!;
+    public Supplier Supplier { get; set; } = null!;
+}