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/notepads/club-work-manager/learnings.md

@@ -136,3 +136,99 @@ _Conventions, patterns, and accumulated wisdom from task execution_
 - Task 3: Populate `realm-export.json` with actual Keycloak realm configuration
 - Task 7: PostgreSQL migrations for Entity Framework Core
 - Task 22: Add backend (Api, Application, Infrastructure services) and frontend to compose file
+
+---
+
+## Task 4: Domain Entities & State Machine (2026-03-03)
+
+### Key Learnings
+
+1. **TDD Approach Workflow**
+   - Write tests FIRST (even when entities don't exist — LSP errors expected)
+   - Create minimal entities to satisfy test requirements
+   - All 12 tests passed on first run after implementation
+   - This validates clean state machine design
+
+2. **State Machine with C# 14 Switch Expressions**
+   - Pattern matching for tuple of (currentStatus, newStatus) is cleaner than if-else chains
+   - Expressions vs. traditional switch: more functional, concise, easier to verify all transitions
+   - Chosen over Dictionary<(status, status), bool> because:
+     - Easier to read and maintain
+     - Compiler can verify exhaustiveness (with `_ => false` fallback)
+     - No runtime lookup overhead
+     - Clear inline state diagram
+
+3. **Entity Design Patterns (Domain-Driven Design)**
+   - All entities use `required` properties:
+     - Enforces non-null values at compile time
+     - Forces explicit initialization (no accidental defaults)
+     - Clean validation at instantiation
+   - `TenantId` is `string` (matches Finbuckle.MultiTenant.ITenantInfo.Id type)
+   - Foreign keys use explicit `Guid` or `Guid?` (not navigation properties yet)
+   - `RowVersion: byte[]?` for optimistic concurrency (EF Core `[Timestamp]` attribute in Task 7)
+
+4. **DateTimeOffset vs DateTime**
+   - Used DateTimeOffset for CreatedAt/UpdatedAt (includes timezone offset)
+   - Better for multi-tenant global apps (know exact UTC moment)
+   - .NET 10 standard for timestamp columns
+   - Avoids timezone confusion across regions
+
+5. **ITenantEntity Interface Pattern**
+   - Explicit interface property (not EF shadow property) allows:
+     - Easy data seeding in tests
+     - LINQ queries without special knowledge
+     - Clear contract in domain code
+   - Marker interface only (no methods) — true DDD boundary
+
+6. **Entity Lifecycle Simplicity**
+   - No domain events (YAGNI for MVP)
+   - No navigation properties (deferred to EF configuration in Task 7)
+   - No validation attributes — EF Fluent API handles in Task 7
+   - State machine is only behavior (business rule enforcement)
+
+7. **Test Structure for TDD**
+   - Helper factory method `CreateWorkItem()` reduces repetition
+   - AAA pattern (Arrange-Act-Assert) clear in test names
+   - Arrange: Create entity with minimal valid state
+   - Act: Call transition method
+   - Assert: Verify state changed or exception thrown
+   - xUnit [Fact] attributes sufficient (no [Theory] needed for now)
+
+8. **Project Structure Observations**
+   - Projects in `backend/` root, not `backend/src/` (deviation from typical convention but works)
+   - Subdirectories: Entities/, Enums/, Interfaces/ (clean separation)
+   - Test mirrors source structure: Domain tests in dedicated folder
+   - Class1.cs stub removed before implementation
+
+### Files Created
+
+- `WorkClub.Domain/Enums/SportType.cs` — 5 values
+- `WorkClub.Domain/Enums/ClubRole.cs` — 4 values  
+- `WorkClub.Domain/Enums/WorkItemStatus.cs` — 5 values
+- `WorkClub.Domain/Interfaces/ITenantEntity.cs` — Marker interface
+- `WorkClub.Domain/Entities/Club.cs` — Basic aggregate root
+- `WorkClub.Domain/Entities/Member.cs` — User representation
+- `WorkClub.Domain/Entities/WorkItem.cs` — Task with state machine
+- `WorkClub.Domain/Entities/Shift.cs` — Volunteer shift
+- `WorkClub.Domain/Entities/ShiftSignup.cs` — Shift registration
+- `WorkClub.Tests.Unit/Domain/WorkItemStatusTests.cs` — 12 tests, all passing
+
+### Build & Test Results
+
+- **Tests**: 12/12 passed (100%)
+  - 5 valid transition tests ✓
+  - 5 invalid transition tests ✓  
+  - 2 CanTransitionTo() method tests ✓
+- **Build**: Release configuration successful
+- **Warnings**: Only Finbuckle version resolution (expected, no errors)
+
+### Next Steps (Tasks 5-7)
+
+- Task 5: Next.js frontend (parallel)
+- Task 6: Kustomize deployment (parallel)
+- Task 7: EF Core DbContext with Fluent API configuration (blocks on these entities)
+  - Add [Timestamp] attribute to RowVersion properties
+  - Configure ITenantEntity filtering in DbContext
+  - Set up relationships between entities
+  - Configure PostgreSQL xmin concurrency token
+