a8a1f6d290
- 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