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/repository/api_key_repo.go

@@ -485,6 +485,7 @@ func groupEntityToService(g *dbent.Group) *service.Group {
 		ModelRoutingEnabled:             g.ModelRoutingEnabled,
 		MCPXMLInject:                    g.McpXMLInject,
 		SupportedModelScopes:            g.SupportedModelScopes,
+		SortOrder:                       g.SortOrder,
 		CreatedAt:                       g.CreatedAt,
 		UpdatedAt:                       g.UpdatedAt,
 	}