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/Entities/WorkItem.cs

@@ -0,0 +1,41 @@
+using WorkClub.Domain.Enums;
+using WorkClub.Domain.Interfaces;
+
+namespace WorkClub.Domain.Entities;
+
+public class WorkItem : ITenantEntity
+{
+    public required Guid Id { get; set; }
+    public required string TenantId { get; set; }
+    public required string Title { get; set; }
+    public string? Description { get; set; }
+    public required WorkItemStatus Status { get; set; }
+    public Guid? AssigneeId { get; set; }
+    public required Guid CreatedById { get; set; }
+    public required Guid ClubId { get; set; }
+    public DateTimeOffset? DueDate { get; set; }
+    public required DateTimeOffset CreatedAt { get; set; }
+    public required DateTimeOffset UpdatedAt { get; set; }
+    public byte[]? RowVersion { get; set; }
+
+    public bool CanTransitionTo(WorkItemStatus newStatus) => (Status, newStatus) switch
+    {
+        (WorkItemStatus.Open, WorkItemStatus.Assigned) => true,
+        (WorkItemStatus.Assigned, WorkItemStatus.InProgress) => true,
+        (WorkItemStatus.InProgress, WorkItemStatus.Review) => true,
+        (WorkItemStatus.Review, WorkItemStatus.Done) => true,
+        (WorkItemStatus.Review, WorkItemStatus.InProgress) => true,
+        _ => false
+    };
+
+    public void TransitionTo(WorkItemStatus newStatus)
+    {
+        if (!CanTransitionTo(newStatus))
+        {
+            throw new InvalidOperationException(
+                $"Cannot transition from {Status} to {newStatus}");
+        }
+
+        Status = newStatus;
+    }
+}