Add NBT support to GRN processing and pricing calculations

Distribution/Models/GoodsReceipt.cs

@@ -36,6 +36,10 @@ namespace Distribution.Models
         [Precision(5, 2)]
         public decimal VatPercent { get; set; }
 
+        /// <summary>GRN-wide default NBT %, applied to any line that doesn't set its own.</summary>
+        [Precision(5, 2)]
+        public decimal NbtPercent { get; set; }
+
         public ICollection<GoodsReceiptItem> Items { get; set; } = [];
     }
 }