Add FlatPaymentController with RESTful payment endpoints

Introduced FlatPaymentController under Distribution.Features.Payment, providing GET and POST endpoints at /api/payments. Supports listing, retrieving, and creating payments with filtering and error handling. Business logic is delegated to PaymentService.

Distribution/Features/Sale/SaleService.cs

@@ -119,6 +119,26 @@ public class SaleService(
         return result;
     }
 
+    public async Task<SaleResponse?> CancelAsync(Guid id, CancellationToken ct = default)
+    {
+        var sale = await repo.GetByIdAsync(id);
+        if (sale is null || !sale.IsActive) return null;
+        if (!await HasStoreScopeAsync(sale.FK_StoreId)) return null;
+        if (sale.Status == SaleStatus.Cancelled)
+            throw new InvalidOperationException("Sale is already cancelled");
+
+        sale.Status = SaleStatus.Cancelled;
+
+        await uow.BeginAsync(ct);
+        try { repo.Update(sale); await uow.CommitAsync(ct); }
+        catch { await uow.RollbackAsync(ct); throw; }
+
+        var order = await repo.GetOrderBySaleIdAsync(id);
+        if (order is null) return null;
+        var items = await orderRepo.GetItemsByOrderIdAsync(order.Id);
+        return ToResponse(sale, order, items);
+    }
+
     public async Task<SaleResponse?> CreateAsync(CreateSaleRequest request, CancellationToken ct = default)
     {
         if (!await HasStoreScopeAsync(request.StoreId)) return null;