feat: drop QuantityOnHand column and StockTransactions table

Final step of removing inventory quantity tracking - see
docs/superpowers/specs/2026-08-01-remove-inventory-quantity-tracking-design.md.
Destructive to any existing on-hand/transaction data; confirmed
acceptable since nothing read it automatically.

CutList.Web/Data/Entities/StockItem.cs

@@ -6,12 +6,10 @@ 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<StockTransaction> Transactions { get; set; } = new List<StockTransaction>();
 }