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/Enums/ClubRole.cs

@@ -0,0 +1,9 @@
+namespace WorkClub.Domain.Enums;
+
+public enum ClubRole
+{
+    Admin = 0,
+    Manager = 1,
+    Member = 2,
+    Viewer = 3
+}

backend/WorkClub.Domain/Enums/SportType.cs

@@ -0,0 +1,10 @@
+namespace WorkClub.Domain.Enums;
+
+public enum SportType
+{
+    Tennis = 0,
+    Cycling = 1,
+    Swimming = 2,
+    Football = 3,
+    Other = 4
+}

backend/WorkClub.Domain/Enums/WorkItemStatus.cs

@@ -0,0 +1,10 @@
+namespace WorkClub.Domain.Enums;
+
+public enum WorkItemStatus
+{
+    Open = 0,
+    Assigned = 1,
+    InProgress = 2,
+    Review = 3,
+    Done = 4
+}