Hook POST /api/dm/:id/members and DELETE /api/dm/:id/members to the
federation outbox so membership changes propagate to peer instances:
- Assign federatedId when a group DM first gains remote members
- Relay member_add events with group metadata and participant-aware routing
- Relay member_remove (leave) events to peer instances
- Relay ownership_transfer when the departing user was the group owner,
updating the federated owner columns accordingly