feat: Support multi-material project parts

Each project part now references its own material, allowing a single
project to use multiple material types. Removes ProjectStockBin entity
since stock is now derived from material stock lengths.

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

CutList.Web/Data/Entities/Material.cs

@@ -11,7 +11,8 @@ public class Material
     public DateTime? UpdatedAt { get; set; }
 
     public ICollection<SupplierStock> SupplierStocks { get; set; } = new List<SupplierStock>();
-    public ICollection<Project> Projects { get; set; } = new List<Project>();
+    public ICollection<MaterialStockLength> StockLengths { get; set; } = new List<MaterialStockLength>();
+    public ICollection<ProjectPart> ProjectParts { get; set; } = new List<ProjectPart>();
 
     public string DisplayName => $"{Shape} - {Size}";
 }

CutList.Web/Data/Entities/ProjectPart.cs

@@ -4,10 +4,12 @@ public class ProjectPart
 {
     public int Id { get; set; }
     public int ProjectId { get; set; }
+    public int MaterialId { get; set; }
     public string Name { get; set; } = string.Empty;
     public decimal LengthInches { get; set; }
     public int Quantity { get; set; } = 1;
     public int SortOrder { get; set; }
 
     public Project Project { get; set; } = null!;
+    public Material Material { get; set; } = null!;
 }

CutList.Web/Data/Entities/ProjectStockBin.cs

@@ -1,13 +0,0 @@
-namespace CutList.Web.Data.Entities;
-
-public class ProjectStockBin
-{
-    public int Id { get; set; }
-    public int ProjectId { get; set; }
-    public decimal LengthInches { get; set; }
-    public int Quantity { get; set; } = -1;
-    public int Priority { get; set; } = 25;
-    public int SortOrder { get; set; }
-
-    public Project Project { get; set; } = null!;
-}