refactor: Update Material and StockItem entities

Material entity changes:
- Shape property now uses MaterialShape enum
- Add Type (MaterialType) and Grade properties
- Add SortOrder for numeric sorting
- Add Dimensions navigation property (1:1)
- Replace ProjectParts with JobParts collection

StockItem entity changes:
- Add QuantityOnHand for inventory tracking
- Add Notes field
- Add Transactions navigation property

DbContext updates:
- Configure MaterialDimensions TPH inheritance
- Add enum-to-string conversions for MaterialShape and MaterialType
- Configure shared column names for TPH properties
- Add indexes on primary dimension columns
- Update all entity relationships for Job model

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

CutList.Web/Data/Entities/StockItem.cs

@@ -6,10 +6,13 @@ public class StockItem
     public int MaterialId { get; set; }
     public decimal LengthInches { get; set; }
     public string? Name { get; set; }
+    public int QuantityOnHand { get; set; } = 0;
+    public string? Notes { 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>();
+    public ICollection<StockTransaction> Transactions { get; set; } = new List<StockTransaction>();
 }