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)

backend/WorkClub.Domain/Interfaces/ITenantEntity.cs

@@ -0,0 +1,13 @@
+namespace WorkClub.Domain.Interfaces;
+
+/// <summary>
+/// Marker interface for tenant-aware entities.
+/// All domain entities must implement this to support multi-tenancy.
+/// </summary>
+public interface ITenantEntity
+{
+    /// <summary>
+    /// Gets or sets the tenant ID (Finbuckle multi-tenant identifier).
+    /// </summary>
+    string TenantId { get; set; }
+}