feat: add preview totals endpoint and DTOs for invoice calculations

Distribution/Features/Invoice/InvoiceDTOs.cs

@@ -1,4 +1,5 @@
-using Distribution.Models;
+using Distribution.Features.ShopPO;
+using Distribution.Models;
 
 namespace Distribution.Features.Invoice;
 
@@ -9,7 +10,14 @@ public record PrintInvoiceRequest(Guid? PrintedByUserId = null);
 
 // InvoiceItemId set = edit/remove/restore an existing line; ItemId set (InvoiceItemId null) = add a new line.
 public record InvoiceItemEdit(Guid? InvoiceItemId, Guid? ItemId, int Quantity, decimal? UnitPrice = null, bool IsRemoved = false);
-public record UpdateInvoiceItemsRequest(List<InvoiceItemEdit> Items, Guid? EditedByUserId = null);
+// DiscountSchemeIds: the percentage-discount scheme chosen in the editor (at most one); null = leave as is.
+public record UpdateInvoiceItemsRequest(List<InvoiceItemEdit> Items, Guid? EditedByUserId = null, List<Guid>? DiscountSchemeIds = null);
+
+public record PreviewLine(Guid ItemId, int Quantity, decimal UnitPrice, bool IsFree = false, bool IsRemoved = false, Guid? SchemeId = null);
+public record PreviewInvoiceTotalsRequest(List<PreviewLine> Items, List<Guid>? DiscountSchemeIds = null);
+// Quantity = how many free units the scheme still entitles for the lines as typed.
+public record FreeEntitlement(Guid SchemeId, int Quantity);
+public record InvoiceTotalsPreview(decimal DiscountAmount, decimal TotalAmount, List<FreeEntitlement> FreeEntitlements);
 
 public record InvoiceVersionItemResponse(Guid? ShopPOItemId, Guid ItemId, string ItemName, int Quantity, decimal UnitPrice, bool IsRemoved);
 
@@ -30,7 +38,10 @@ public record InvoiceItemResponse(
     int? OrderedQuantity,
     int Quantity,
     decimal UnitPrice,
-    bool IsRemoved);
+    bool IsRemoved,
+    bool IsFree = false,
+    Guid? SchemeId = null,
+    string? SchemeName = null);
 
 public record InvoiceResponse(
     Guid Id,
@@ -49,6 +60,7 @@ public record InvoiceResponse(
     decimal PaidAmount,
     bool IsActive,
     DateTime CreatedAt,
-    List<InvoiceItemResponse> Items);
+    List<InvoiceItemResponse> Items,
+    List<ShopPODiscountResponse>? Discounts = null);
 
 public record OutstandingPaymentsResponse(int InvoiceCount, decimal OutstandingAmount);