feat(workspace): add WorkspaceInvitation entity

- Add InvitationStatus enum (pending/accepted/declined/expired)
- Add WorkspaceInvitation entity to track invitation lifecycle
- Store inviter, invitee email, role, and invitation token
- Track status, expiration, and acceptance time
- Foundation for workspace invitation system

Phase 4 Task 11/68 completed

packages/domain/entities.py

@@ -60,6 +60,14 @@ class WorkspaceMemberRole(StrEnum):
     VIEWER = "viewer"  # 查看者（只读）
 
 
+class InvitationStatus(StrEnum):
+    """邀请状态"""
+    PENDING = "pending"  # 待处理
+    ACCEPTED = "accepted"  # 已接受
+    DECLINED = "declined"  # 已拒绝
+    EXPIRED = "expired"  # 已过期
+
+
 @dataclass(slots=True)
 class WorkspaceMember:
     """工作空间成员"""
@@ -71,6 +79,21 @@ class WorkspaceMember:
     joined_at: datetime = field(default_factory=lambda: datetime.now(timezone.utc))
 
 
+@dataclass(slots=True)
+class WorkspaceInvitation:
+    """工作空间邀请"""
+    id: str
+    workspace_id: str
+    inviter_user_id: str  # 邀请人
+    invitee_email: str  # 被邀请人邮箱
+    role: str  # 邀请角色
+    invitation_token: str  # 邀请令牌
+    status: str = "pending"  # pending, accepted, declined, expired
+    expires_at: datetime | None = None  # 过期时间
+    accepted_at: datetime | None = None  # 接受时间
+    created_at: datetime = field(default_factory=lambda: datetime.now(timezone.utc))
+
+
 @dataclass(slots=True)
 class Project:
     id: str