feat(domain): add core entities — Club, Member, WorkItem, Shift with state machine

- Create domain entities in WorkClub.Domain/Entities: Club, Member, WorkItem, Shift, ShiftSignup
- Implement enums: SportType, ClubRole, WorkItemStatus
- Add ITenantEntity interface for multi-tenancy support
- Implement state machine validation on WorkItem with C# 14 switch expressions
- Valid transitions: Open→Assigned→InProgress→Review→Done, Review→InProgress (rework)
- All invalid transitions throw InvalidOperationException
- TDD approach: Write tests first, 12/12 passing
- Use required properties with explicit Guid/Guid? for foreign keys
- DateTimeOffset for timestamps (timezone-aware, multi-tenant friendly)
- RowVersion byte[] for optimistic concurrency control
- No navigation properties yet (deferred to EF Core task)
- No domain events or validation attributes (YAGNI for MVP)

infra/k8s/base/kustomization.yaml

@@ -0,0 +1,14 @@
+apiVersion: kustomize.config.k8s.io/v1beta1
+kind: Kustomization
+
+resources:
+  - backend-deployment.yaml
+  - backend-service.yaml
+  - frontend-deployment.yaml
+  - frontend-service.yaml
+  - postgres-statefulset.yaml
+  - postgres-service.yaml
+  - keycloak-deployment.yaml
+  - keycloak-service.yaml
+  - configmap.yaml
+  - ingress.yaml