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)

.sisyphus/evidence/task-6-resource-names.txt

@@ -0,0 +1,26 @@
+Full Resource List from kustomize build
+========================================
+
+Deployments:
+  - workclub-api (port 8080, dotnet-api service)
+  - workclub-frontend (port 3000, nextjs service)
+  - workclub-keycloak (port 8080, auth service)
+
+StatefulSet:
+  - workclub-postgres (port 5432, database service)
+
+Services (ClusterIP):
+  - workclub-api (selector: app=workclub-api)
+  - workclub-frontend (selector: app=workclub-frontend)
+  - workclub-keycloak (selector: app=workclub-keycloak)
+  - workclub-postgres (selector: app=workclub-postgres)
+  - workclub-postgres-headless (clusterIP: None, for StatefulSet DNS)
+
+ConfigMaps:
+  - workclub-config (application settings: log-level, cors-origins, api-base-url, etc.)
+  - postgres-init (initialization script for database setup)
+
+Ingress:
+  - workclub-ingress (path-based routing: / → frontend, /api → backend)
+
+All resources created with consistent workclub- prefix