Introduced meeting-specific conversation lifecycle events: `conversation.create-meeting` and `conversation.delete-meeting`. When a conversation of type meeting (`group_conv_type: "meeting"`) is created or deleted, clients receive these instead of `conversation.create` / `conversation.delete`. The payloads are identical to their non-meeting counterparts (`conversation.delete-meeting`, like `conversation.delete`, carries no `data`); only the event `type` differs, so clients can handle meetings distinctly. (WPB-26626)
