fix(group): 管理员清空分组描述时正确持久化

UpdateGroup 之前用 `if input.Description != ""` 判空，
把"未提供"和"显式置空"混为一谈，导致管理员在分组编辑表单
里清空备注后保存无效。

将 UpdateGroupRequest / UpdateGroupInput 的 Description 改为
*string：nil 表示未提供（保持原值），"" 表示显式清空。

backend/internal/handler/admin/group_handler.go

@@ -123,7 +123,7 @@ type CreateGroupRequest struct {
 // UpdateGroupRequest represents update group request
 type UpdateGroupRequest struct {
 	Name             string             `json:"name"`
-	Description      string             `json:"description"`
+	Description      *string            `json:"description"`
 	Platform         string             `json:"platform" binding:"omitempty,oneof=anthropic openai gemini antigravity"`
 	RateMultiplier   *float64           `json:"rate_multiplier"`
 	IsExclusive      *bool              `json:"is_exclusive"`

backend/internal/service/admin_service.go

@@ -230,7 +230,7 @@ type CreateGroupInput struct {
 
 type UpdateGroupInput struct {
 	Name             string
-	Description      string
+	Description      *string
 	Platform         string
 	RateMultiplier   *float64 // 使用指针以支持设置为0
 	IsExclusive      *bool
@@ -1924,8 +1924,8 @@ func (s *adminServiceImpl) UpdateGroup(ctx context.Context, id int64, input *Upd
 	if input.Name != "" {
 		group.Name = input.Name
 	}
-	if input.Description != "" {
+	if input.Description != nil {
-		group.Description = input.Description
+		group.Description = *input.Description
 	}
 	if input.Platform != "" {
 		group.Platform = input.Platform

backend/internal/service/admin_service_group_test.go

@@ -280,8 +280,9 @@ func TestAdminService_UpdateGroup_PreservesImageGenerationControlsWhenOmitted(t
 	repo := &groupRepoStubForAdmin{getByID: existingGroup}
 	svc := &adminServiceImpl{groupRepo: repo}
 
+	updatedDesc := "updated"
 	group, err := svc.UpdateGroup(context.Background(), 1, &UpdateGroupInput{
-		Description: "updated",
+		Description: &updatedDesc,
 	})
 	require.NoError(t, err)
 	require.NotNil(t, group)
@@ -291,6 +292,45 @@ func TestAdminService_UpdateGroup_PreservesImageGenerationControlsWhenOmitted(t
 	require.InDelta(t, 0.5, repo.updated.ImageRateMultiplier, 1e-12)
 }
 
+func TestAdminService_UpdateGroup_ClearsDescriptionWhenEmptyString(t *testing.T) {
+	existingGroup := &Group{
+		ID:          1,
+		Name:        "existing-group",
+		Description: "Auto-created default group",
+		Platform:    PlatformOpenAI,
+		Status:      StatusActive,
+	}
+	repo := &groupRepoStubForAdmin{getByID: existingGroup}
+	svc := &adminServiceImpl{groupRepo: repo}
+
+	empty := ""
+	_, err := svc.UpdateGroup(context.Background(), 1, &UpdateGroupInput{
+		Description: &empty,
+	})
+	require.NoError(t, err)
+	require.NotNil(t, repo.updated)
+	require.Equal(t, "", repo.updated.Description, "empty string should clear description")
+}
+
+func TestAdminService_UpdateGroup_PreservesDescriptionWhenNil(t *testing.T) {
+	existingGroup := &Group{
+		ID:          1,
+		Name:        "existing-group",
+		Description: "keep me",
+		Platform:    PlatformOpenAI,
+		Status:      StatusActive,
+	}
+	repo := &groupRepoStubForAdmin{getByID: existingGroup}
+	svc := &adminServiceImpl{groupRepo: repo}
+
+	_, err := svc.UpdateGroup(context.Background(), 1, &UpdateGroupInput{
+		Description: nil,
+	})
+	require.NoError(t, err)
+	require.NotNil(t, repo.updated)
+	require.Equal(t, "keep me", repo.updated.Description, "nil should preserve existing description")
+}
+
 func TestAdminService_UpdateGroup_RejectsNegativeImageRateMultiplier(t *testing.T) {
 	existingGroup := &Group{
 		ID:                  1,