refactor: full-stack restructure with multi-tenancy, workspace management, and K8s diagnostics

- Add Workspace domain (entity, repository, service, handler, DTO)
- Add multi-tenant K8s client with tenant binding and quota management
- Add K8s diagnostics client (instance diagnostics)
- Add authorization middleware (authz package)
- Restructure frontend to feature-based architecture (features/)
- Add User Management page in configuration
- Add AccessDenied page and route guards
- Refactor shared components (form inputs, layout, UI)
- Update Tailwind config for new design system
- Add comprehensive documentation (docs/, tasks/, plans)
- Improve cluster service with better kubeconfig handling
- Add tests for crypto, config, helm client, tenant binding

backend/internal/domain/repository/instance_diagnostics_client.go

@@ -0,0 +1,11 @@
+package repository
+
+import (
+	"context"
+
+	"github.com/ocdp/cluster-service/internal/domain/entity"
+)
+
+type InstanceDiagnosticsClient interface {
+	GetDiagnostics(ctx context.Context, cluster *entity.Cluster, instance *entity.Instance, tailLines int64) (*entity.InstanceDiagnostics, error)
+}