fix: scale replicas in response, K8s metrics client, quota precheck, auth tests

- Add GetMetrics method to MetricsClient interface and implement cluster metrics API
- Add QuotaPrecheck service for validating resource quotas before deployment
- Add auth DTO with role/permission models and auth handler tests
- Add instance diagnostics: mounted NFS volumes, labels, annotations in pod diagnostics
- Update workspace handler with GetWorkspace endpoint and shared-user list
- Fix monitoring handler to use correct service method name
- Add tail_lines fallback in instance handler for snake_case query params
- Update nginx config for SSE log streaming support (no buffering)
- Add comprehensive test coverage: auth_service_test, auth_handler_test,
  auth_dto_test, metrics_client_test, quota_precheck_test
- Update error messages for quota validation and instance operations
- ModifyModal: fix YAML lineWidth:0, modified keys summary, delta-only submit
- InstanceCard: correctly disable scale-minus when replicas <= 0
- SidebarLayout: add hover transition for sidebar items
- Update todo.md and lessons.md with latest fixes

backend/internal/domain/entity/errors.go

@@ -43,6 +43,9 @@ var (
 	ErrValuesSchemaNotFound = errors.New("values schema not found")
 
 	// Workspace errors
-	ErrWorkspaceNotFound = errors.New("workspace not found")
-	ErrWorkspaceExists   = errors.New("workspace already exists")
+	ErrWorkspaceNotFound          = errors.New("workspace not found")
+	ErrWorkspaceExists            = errors.New("workspace already exists")
+	ErrWorkspaceNamespaceConflict = errors.New("workspace namespace conflict")
+	ErrUserHasInstances           = errors.New("user has active instances")
+	ErrProtectedNamespace         = errors.New("protected namespace")
 )