feat(admin): add drag-and-drop group sort order

- Add `sort_order` field to groups table with migration
- Add `PUT /api/v1/admin/groups/sort-order` API for batch update
- Implement drag-and-drop UI using vue-draggable-plus
- All queries now order groups by sort_order
- Add i18n support (en/zh) for sort-related UI text
- Update test stubs to satisfy new interface methods

backend/internal/service/group.go

@@ -45,6 +45,9 @@ type Group struct {
 	// 可选值: claude, gemini_text, gemini_image
 	SupportedModelScopes []string
 
+	// 分组排序
+	SortOrder int
+
 	CreatedAt time.Time
 	UpdatedAt time.Time