refactor: Replace SupplierStock with StockItem/SupplierOffering model

- Remove SupplierStock entity
- Update Material navigation from SupplierStocks to StockItems
- Update Supplier navigation from Stocks to Offerings
- Update ApplicationDbContext with new DbSets and configurations
- Add unique constraints: StockItem(MaterialId, LengthInches) and
  SupplierOffering(SupplierId, StockItemId)

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

CutList.Web/Data/Entities/Material.cs

@@ -10,7 +10,7 @@ public class Material
     public DateTime CreatedAt { get; set; } = DateTime.UtcNow;
     public DateTime? UpdatedAt { get; set; }
 
-    public ICollection<SupplierStock> SupplierStocks { get; set; } = new List<SupplierStock>();
+    public ICollection<StockItem> StockItems { get; set; } = new List<StockItem>();
     public ICollection<MaterialStockLength> StockLengths { get; set; } = new List<MaterialStockLength>();
     public ICollection<ProjectPart> ProjectParts { get; set; } = new List<ProjectPart>();
 

CutList.Web/Data/Entities/Supplier.cs

@@ -9,5 +9,5 @@ public class Supplier
     public bool IsActive { get; set; } = true;
     public DateTime CreatedAt { get; set; } = DateTime.UtcNow;
 
-    public ICollection<SupplierStock> Stocks { get; set; } = new List<SupplierStock>();
+    public ICollection<SupplierOffering> Offerings { get; set; } = new List<SupplierOffering>();
 }

CutList.Web/Data/Entities/SupplierStock.cs

@@ -1,15 +0,0 @@
-namespace CutList.Web.Data.Entities;
-
-public class SupplierStock
-{
-    public int Id { get; set; }
-    public int SupplierId { get; set; }
-    public int MaterialId { get; set; }
-    public decimal LengthInches { get; set; }
-    public decimal? Price { get; set; }
-    public string? Notes { get; set; }
-    public bool IsActive { get; set; } = true;
-
-    public Supplier Supplier { get; set; } = null!;
-    public Material Material { get; set; } = null!;
-}