schema: rm is_dm, rename name→userinfo (json_flat), fix join_msg format

- Remove is_dm column from channels table (dm detection by member count)
- Rename member name column to userinfo — extensible json_flat format
- Add json_flat.c/h library for compact JSON serialization
- Fix join_sig message format: use ch_x25519+ch_ed25519 instead of channel_id string
- Remove name from join_msg body (now in userinfo via json_flat)
- Update chat_core, chat_sync, member_sync, db_manager to new format
- Add topo_strategy.txt — архитектурные заметки по топологии

tools/chatgui/src/mainwindow.cpp

@@ -477,7 +477,6 @@ void MainWindow::onCreateGroupRequested() {
     snprintf(req->channel_id, sizeof(req->channel_id), "%s", qPrintable(channelId));
     snprintf(req->name, sizeof(req->name), "%s", qPrintable(name));
     req->owner_node_id = ownerNodeId;
-    req->is_dm = 0;
     memcpy(req->x25519_pubkey, x25519_pub, 32);
     memcpy(req->x25519_privkey, x25519_priv, 32);
     memcpy(req->ed25519_pubkey, ed25519_pub, 32);