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/Class1.cs

@@ -1,6 +0,0 @@
-namespace WorkClub.Domain;
-
-public class Class1
-{
-
-}

backend/WorkClub.Domain/Entities/Club.cs

@@ -0,0 +1,15 @@
+using WorkClub.Domain.Enums;
+using WorkClub.Domain.Interfaces;
+
+namespace WorkClub.Domain.Entities;
+
+public class Club : ITenantEntity
+{
+    public required Guid Id { get; set; }
+    public required string TenantId { get; set; }
+    public required string Name { get; set; }
+    public required SportType SportType { get; set; }
+    public string? Description { get; set; }
+    public required DateTimeOffset CreatedAt { get; set; }
+    public required DateTimeOffset UpdatedAt { get; set; }
+}

backend/WorkClub.Domain/Entities/Member.cs

@@ -0,0 +1,17 @@
+using WorkClub.Domain.Enums;
+using WorkClub.Domain.Interfaces;
+
+namespace WorkClub.Domain.Entities;
+
+public class Member : ITenantEntity
+{
+    public required Guid Id { get; set; }
+    public required string TenantId { get; set; }
+    public required string ExternalUserId { get; set; }
+    public required string DisplayName { get; set; }
+    public required string Email { get; set; }
+    public required ClubRole Role { get; set; }
+    public required Guid ClubId { get; set; }
+    public required DateTimeOffset CreatedAt { get; set; }
+    public required DateTimeOffset UpdatedAt { get; set; }
+}

backend/WorkClub.Domain/Entities/Shift.cs

@@ -0,0 +1,20 @@
+using WorkClub.Domain.Interfaces;
+
+namespace WorkClub.Domain.Entities;
+
+public class Shift : 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 string? Location { get; set; }
+    public required DateTimeOffset StartTime { get; set; }
+    public required DateTimeOffset EndTime { get; set; }
+    public int Capacity { get; set; } = 1;
+    public required Guid ClubId { get; set; }
+    public required Guid CreatedById { get; set; }
+    public required DateTimeOffset CreatedAt { get; set; }
+    public required DateTimeOffset UpdatedAt { get; set; }
+    public byte[]? RowVersion { get; set; }
+}

backend/WorkClub.Domain/Entities/ShiftSignup.cs

@@ -0,0 +1,12 @@
+using WorkClub.Domain.Interfaces;
+
+namespace WorkClub.Domain.Entities;
+
+public class ShiftSignup : ITenantEntity
+{
+    public required Guid Id { get; set; }
+    public required string TenantId { get; set; }
+    public required Guid ShiftId { get; set; }
+    public required Guid MemberId { get; set; }
+    public required DateTimeOffset SignedUpAt { get; set; }
+}

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;
+    }
+}

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
+}

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; }
+}

backend/WorkClub.Tests.Unit/Domain/WorkItemStatusTests.cs

@@ -0,0 +1,168 @@
+using WorkClub.Domain.Entities;
+using WorkClub.Domain.Enums;
+
+namespace WorkClub.Tests.Unit.Domain;
+
+public class WorkItemStatusTests
+{
+    private static WorkItem CreateWorkItem(WorkItemStatus status = WorkItemStatus.Open)
+    {
+        return new WorkItem
+        {
+            Id = Guid.NewGuid(),
+            TenantId = "test-tenant",
+            Title = "Test Work Item",
+            Status = status,
+            CreatedById = Guid.NewGuid(),
+            ClubId = Guid.NewGuid(),
+            CreatedAt = DateTimeOffset.UtcNow,
+            UpdatedAt = DateTimeOffset.UtcNow
+        };
+    }
+
+    [Fact]
+    public void Open_ToAssigned_Succeeds()
+    {
+        // Arrange
+        var workItem = CreateWorkItem(WorkItemStatus.Open);
+
+        // Act
+        workItem.TransitionTo(WorkItemStatus.Assigned);
+
+        // Assert
+        Assert.Equal(WorkItemStatus.Assigned, workItem.Status);
+    }
+
+    [Fact]
+    public void Assigned_ToInProgress_Succeeds()
+    {
+        // Arrange
+        var workItem = CreateWorkItem(WorkItemStatus.Assigned);
+
+        // Act
+        workItem.TransitionTo(WorkItemStatus.InProgress);
+
+        // Assert
+        Assert.Equal(WorkItemStatus.InProgress, workItem.Status);
+    }
+
+    [Fact]
+    public void InProgress_ToReview_Succeeds()
+    {
+        // Arrange
+        var workItem = CreateWorkItem(WorkItemStatus.InProgress);
+
+        // Act
+        workItem.TransitionTo(WorkItemStatus.Review);
+
+        // Assert
+        Assert.Equal(WorkItemStatus.Review, workItem.Status);
+    }
+
+    [Fact]
+    public void Review_ToDone_Succeeds()
+    {
+        // Arrange
+        var workItem = CreateWorkItem(WorkItemStatus.Review);
+
+        // Act
+        workItem.TransitionTo(WorkItemStatus.Done);
+
+        // Assert
+        Assert.Equal(WorkItemStatus.Done, workItem.Status);
+    }
+
+    [Fact]
+    public void Review_ToInProgress_Succeeds()
+    {
+        // Arrange
+        var workItem = CreateWorkItem(WorkItemStatus.Review);
+
+        // Act
+        workItem.TransitionTo(WorkItemStatus.InProgress);
+
+        // Assert
+        Assert.Equal(WorkItemStatus.InProgress, workItem.Status);
+    }
+
+    [Fact]
+    public void Open_ToDone_Throws()
+    {
+        // Arrange
+        var workItem = CreateWorkItem(WorkItemStatus.Open);
+
+        // Act & Assert
+        Assert.Throws<InvalidOperationException>(() =>
+            workItem.TransitionTo(WorkItemStatus.Done));
+    }
+
+    [Fact]
+    public void Open_ToInProgress_Throws()
+    {
+        // Arrange
+        var workItem = CreateWorkItem(WorkItemStatus.Open);
+
+        // Act & Assert
+        Assert.Throws<InvalidOperationException>(() =>
+            workItem.TransitionTo(WorkItemStatus.InProgress));
+    }
+
+    [Fact]
+    public void Assigned_ToDone_Throws()
+    {
+        // Arrange
+        var workItem = CreateWorkItem(WorkItemStatus.Assigned);
+
+        // Act & Assert
+        Assert.Throws<InvalidOperationException>(() =>
+            workItem.TransitionTo(WorkItemStatus.Done));
+    }
+
+    [Fact]
+    public void InProgress_ToOpen_Throws()
+    {
+        // Arrange
+        var workItem = CreateWorkItem(WorkItemStatus.InProgress);
+
+        // Act & Assert
+        Assert.Throws<InvalidOperationException>(() =>
+            workItem.TransitionTo(WorkItemStatus.Open));
+    }
+
+    [Fact]
+    public void Done_ToAnyStatus_Throws()
+    {
+        // Arrange
+        var workItem = CreateWorkItem(WorkItemStatus.Done);
+
+        // Act & Assert
+        Assert.Throws<InvalidOperationException>(() =>
+            workItem.TransitionTo(WorkItemStatus.Open));
+        Assert.Throws<InvalidOperationException>(() =>
+            workItem.TransitionTo(WorkItemStatus.Assigned));
+        Assert.Throws<InvalidOperationException>(() =>
+            workItem.TransitionTo(WorkItemStatus.InProgress));
+        Assert.Throws<InvalidOperationException>(() =>
+            workItem.TransitionTo(WorkItemStatus.Review));
+    }
+
+    [Fact]
+    public void CanTransitionTo_ValidTransition_ReturnsTrue()
+    {
+        // Arrange
+        var workItem = CreateWorkItem(WorkItemStatus.Open);
+
+        // Act & Assert
+        Assert.True(workItem.CanTransitionTo(WorkItemStatus.Assigned));
+    }
+
+    [Fact]
+    public void CanTransitionTo_InvalidTransition_ReturnsFalse()
+    {
+        // Arrange
+        var workItem = CreateWorkItem(WorkItemStatus.Open);
+
+        // Act & Assert
+        Assert.False(workItem.CanTransitionTo(WorkItemStatus.Done));
+    }
+}

backend/WorkClub.Tests.Unit/UnitTest1.cs

@@ -1,10 +0,0 @@
-namespace WorkClub.Tests.Unit;
-
-public class UnitTest1
-{
-    [Fact]
-    public void Test1()
-    {
-
-    }
-}