feat: Add stock transaction tracking system

- Add StockTransaction entity for audit trail
- Track received, used, adjusted, scrapped, and returned stock
- Include unit price tracking for cost analysis
- Link transactions to jobs and suppliers

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

CutList.Web/Data/Entities/StockTransaction.cs

@@ -0,0 +1,27 @@
+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 int? SupplierId { get; set; }
+    public decimal? UnitPrice { 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 Supplier? Supplier { get; set; }
+}
+
+public enum StockTransactionType
+{
+    Received,
+    Used,
+    Adjustment,
+    Scrapped,
+    Returned
+}