fix(build): add missing System.Linq import in DiscountOrderService

Fixes CI compile error CS1061 on DbSet.Where; remove duplicate using in UserOrderService.

Co-authored-by: Cursor <cursoragent@cursor.com>

src/CMSMicroservice.WebApi/Services/DiscountOrderService.cs

@@ -17,6 +17,7 @@ using Mapster;
 using MediatR;
 using Microsoft.EntityFrameworkCore;
 using Microsoft.Extensions.Logging;
+using System.Linq;
 
 namespace CMSMicroservice.WebApi.Services;
 

src/CMSMicroservice.WebApi/Services/UserOrderService.cs

@@ -20,7 +20,6 @@ using CMSMicroservice.Protobuf.Protos;
 using MediatR;
 using Mapster;
 using Microsoft.EntityFrameworkCore;
-using System.Linq;
 
 namespace CMSMicroservice.WebApi.Services;