- Implement AcceptInvitationUseCase with validation
- Check invitation status, expiration, and email match
- Auto-create WorkspaceMember on accept
- Handle case when user is already a member
- Implement DeclineInvitationUseCase to reject invitations
- Update invitation status (accepted/declined/expired)
- Add 9 comprehensive unit tests (all passed)
Phase 4 Task 14/68 completed