feat: Add MaterialStockLength entity for inventory tracking

Introduces a new entity to track available stock lengths per material,
enabling in-stock vs. purchase-needed distinction during optimization.

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

CutList.Web/Data/Entities/MaterialStockLength.cs

@@ -0,0 +1,13 @@
+namespace CutList.Web.Data.Entities;
+
+public class MaterialStockLength
+{
+    public int Id { get; set; }
+    public int MaterialId { get; set; }
+    public decimal LengthInches { get; set; }
+    public int Quantity { get; set; } = 0;
+    public string? Notes { get; set; }
+    public bool IsActive { get; set; } = true;
+
+    public Material Material { get; set; } = null!;
+}