feat(workspace): add WorkspaceMember entity with role-based access

- Add WorkspaceMemberRole enum (owner/admin/member/viewer)
- Add WorkspaceMember entity to track user membership in workspace
- Track invited_by for audit trail
- Track joined_at for membership timeline
- Foundation for multi-tenant permission system

Phase 4 Task 10/68 completed

packages/domain/entities.py

@@ -52,6 +52,25 @@ class Workspace:
     created_at: datetime = field(default_factory=lambda: datetime.now(timezone.utc))
 
 
+class WorkspaceMemberRole(StrEnum):
+    """工作空间成员角色"""
+    OWNER = "owner"  # 所有者（创建者，唯一）
+    ADMIN = "admin"  # 管理员（可管理成员和项目）
+    MEMBER = "member"  # 成员（可创建和编辑项目）
+    VIEWER = "viewer"  # 查看者（只读）
+
+
+@dataclass(slots=True)
+class WorkspaceMember:
+    """工作空间成员"""
+    id: str
+    workspace_id: str
+    user_id: str
+    role: str  # owner, admin, member, viewer
+    invited_by: str | None = None  # 邀请人 user_id
+    joined_at: datetime = field(default_factory=lambda: datetime.now(timezone.utc))
+
+
 @dataclass(slots=True)
 class Project:
     id: str