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/workspace_repository.go

@@ -0,0 +1,26 @@
+package repository
+
+import (
+	"context"
+
+	"github.com/ocdp/cluster-service/internal/domain/entity"
+)
+
+type WorkspaceRepository interface {
+	Create(ctx context.Context, workspace *entity.Workspace) error
+	GetByID(ctx context.Context, id string) (*entity.Workspace, error)
+	GetByName(ctx context.Context, name string) (*entity.Workspace, error)
+	Update(ctx context.Context, workspace *entity.Workspace) error
+	List(ctx context.Context) ([]*entity.Workspace, error)
+}
+
+type WorkspaceClusterBindingRepository interface {
+	Upsert(ctx context.Context, binding *entity.WorkspaceClusterBinding) error
+	Get(ctx context.Context, workspaceID, clusterID string) (*entity.WorkspaceClusterBinding, error)
+	Delete(ctx context.Context, workspaceID, clusterID string) error
+}
+
+type AuditLogRepository interface {
+	Create(ctx context.Context, log *entity.AuditLog) error
+	ListByWorkspace(ctx context.Context, workspaceID string, limit int) ([]*entity.AuditLog, error)
+}