fix(backend): 订单列表 user_name 在 real_name 为空时回退 username

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

backend/internal/service/pay.go

@@ -741,7 +741,11 @@ func (s *PayService) ListPurchases(shopID uint64, page, pageSize int, status str
 			return nil, err
 		}
 		for _, u := range users {
-			names[u.ID] = u.RealName
+			if u.RealName != "" {
+				names[u.ID] = u.RealName
+			} else {
+				names[u.ID] = u.Username
+			}
 		}
 	}
 

backend/internal/service/pay_test.go

@@ -942,3 +942,25 @@ func TestListPurchases_UserNameJoin(t *testing.T) {
 	assert.Equal(t, "王老板", byOtn["un-known"].UserName)
 	assert.Empty(t, byOtn["un-missing"].UserName, "查不到用户留空")
 }
+
+// TestListPurchases_UserNameFallbackToUsername：real_name 为空（用户创建接口该字段可选）时，
+// user_name 必须回退 username，不能显示空白。
+func TestListPurchases_UserNameFallbackToUsername(t *testing.T) {
+	db := testutil.SetupTestDB()
+	svc := newTestPaySvc(db, "http://pay.invalid")
+	shop := testutil.CreateTestShop(db, "PAY108")
+	user := testutil.CreateTestUser(db, shop.ID, "noreal", "pass", "admin")
+	// CreateTestUser 默认会填 real_name，显式置空才能命中回退分支。
+	require.NoError(t, db.Model(user).Update("real_name", "").Error)
+
+	base := time.Now().Add(-time.Hour)
+	createPurchaseFull(t, db, shop.ID, user.ID, "annual_pro", 599900, "un-noreal", "paid", "", base)
+
+	list, err := svc.ListPurchases(shop.ID, 1, 20, "")
+	require.NoError(t, err)
+	byOtn := map[string]PurchaseListItem{}
+	for _, item := range list.Items {
+		byOtn[item.OutTradeNo] = item
+	}
+	assert.Equal(t, "noreal", byOtn["un-noreal"].UserName, "real_name 为空时应回退 username")
+}