Add three new event processors to the relay endpoint for handling
inbound group DM membership changes from peer instances:
- processMemberAddEvent: handles member additions with channel bootstrap
(creates the local channel from group metadata on first contact)
- processMemberRemoveEvent: handles member removals with read state
cleanup and soft-delete GC when no local members remain
- processOwnershipTransferEvent: updates local channel owner tracking
All three are wired into the relay endpoint's event dispatch switch.
Jannis Braun
242004e8ee