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>();
 }

CutList.Web/Data/Entities/StockTransaction.cs

@@ -1,24 +0,0 @@
-namespace CutList.Web.Data.Entities;
-
-public class StockTransaction
-{
-    public int Id { get; set; }
-    public int StockItemId { get; set; }
-    public int Quantity { get; set; }
-    public StockTransactionType Type { get; set; }
-    public int? JobId { get; set; }
-    public string? Notes { get; set; }
-    public DateTime CreatedAt { get; set; } = DateTime.UtcNow;
-
-    public StockItem StockItem { get; set; } = null!;
-    public Job? Job { get; set; }
-}
-
-public enum StockTransactionType
-{
-    Received,
-    Used,
-    Adjustment,
-    Scrapped,
-    Returned
-}