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)

infra/k8s/base/postgres-statefulset.yaml

@@ -0,0 +1,91 @@
+apiVersion: apps/v1
+kind: StatefulSet
+metadata:
+  name: workclub-postgres
+  labels:
+    app: workclub-postgres
+    component: database
+spec:
+  serviceName: workclub-postgres-headless
+  replicas: 1
+  selector:
+    matchLabels:
+      app: workclub-postgres
+  template:
+    metadata:
+      labels:
+        app: workclub-postgres
+        component: database
+    spec:
+      containers:
+      - name: postgres
+        image: postgres:16-alpine
+        imagePullPolicy: IfNotPresent
+        ports:
+        - name: postgresql
+          containerPort: 5432
+          protocol: TCP
+        livenessProbe:
+          exec:
+            command:
+            - /bin/sh
+            - -c
+            - pg_isready -U app -d workclub
+          initialDelaySeconds: 10
+          periodSeconds: 15
+          timeoutSeconds: 5
+          failureThreshold: 3
+        
+        readinessProbe:
+          exec:
+            command:
+            - /bin/sh
+            - -c
+            - pg_isready -U app -d workclub
+          initialDelaySeconds: 5
+          periodSeconds: 10
+          timeoutSeconds: 5
+          failureThreshold: 2
+        
+        resources:
+          requests:
+            cpu: 100m
+            memory: 256Mi
+          limits:
+            cpu: 500m
+            memory: 512Mi
+        
+        env:
+        - name: POSTGRES_DB
+          value: workclub
+        - name: POSTGRES_USER
+          value: app
+        - name: POSTGRES_PASSWORD
+          valueFrom:
+            secretKeyRef:
+              name: workclub-secrets
+              key: postgres-password
+        volumeMounts:
+        - name: postgres-data
+          mountPath: /var/lib/postgresql/data
+          subPath: postgres
+        - name: postgres-init
+          mountPath: /docker-entrypoint-initdb.d
+      
+      volumes:
+        - name: postgres-init
+          configMap:
+            name: postgres-init
+            items:
+            - key: init.sql
+              path: init.sql
+  volumeClaimTemplates:
+  - metadata:
+      name: postgres-data
+    spec:
+      accessModes:
+      - ReadWriteOnce
+      storageClassName: standard
+      resources:
+        requests:
+          storage: 10Gi