add day-end and change item model

Backend/ERPCore/Domain/DocumentTypes.cs

@@ -20,6 +20,7 @@ public static class DocumentTypes
     public const string SalesInvoice = "SI";
     public const string SalesSlip = "SSL";
     public const string BundleSale = "BND";
+    public const string BundleSaleTemplate = "TPL";
     public const string SalesReturn = "SRET";
 
     /// <summary>Cashier day-end close-out (Sales) — <c>DEND-2026-00001</c>.</summary>

Backend/ERPCore/Domain/Entities/Item.cs

@@ -44,11 +44,12 @@ public class Item
     public string? TaxClass { get; set; }
 
     /// <summary>
-    /// Optional fixed selling price used by Sales only. <c>null</c> means "use stock value"
-    /// (the item is sold at its FIFO stock cost at sale time); a value is the fixed sale price.
-    /// Never enters costing/GRN/FIFO (docs/10 Part C.1, C.9).
+    /// Optional selling price range and flat price used by Sales only.
+    /// <c>null</c> values mean the field is not set. These values never enter costing/GRN/FIFO.
     /// </summary>
-    public decimal? SalePrice { get; set; }
+    public decimal? SellingMinPrice { get; set; }
+    public decimal? SellingMaxPrice { get; set; }
+    public decimal? SellingFlatPrice { get; set; }
 
     /// <summary>
     /// How much one pack holds, as the user entered it — <c>500</c> with