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/db/db_manager.cpp

@@ -95,7 +95,7 @@ QByteArray DbManager::colBlob(sqlite3_stmt* stmt, int col) {
 QList<ChannelRow> DbManager::getChannels() const {
     QList<ChannelRow> list;
     sqlite3_stmt* stmt = prepareOrNull(
-        "SELECT channel_id, name, owner_node_id, is_dm,"
+        "SELECT channel_id, name, owner_node_id,"
         " x25519_pubkey, x25519_privkey, ed25519_pubkey, ed25519_privkey, signature,"
         " last_read_msg_id, last_pos_msg_id, created_at"
         " FROM channels ORDER BY created_at ASC");
@@ -105,15 +105,14 @@ QList<ChannelRow> DbManager::getChannels() const {
         r.channelId       = colText(stmt, 0);
         r.name            = colText(stmt, 1);
         r.ownerNodeId     = (quint64)sqlite3_column_int64(stmt, 2);
-        r.isDm            = sqlite3_column_int(stmt, 3) != 0;
-        r.x25519_pubkey   = colBlob(stmt, 4);
-        r.x25519_privkey  = colBlob(stmt, 5);
-        r.ed25519_pubkey  = colBlob(stmt, 6);
-        r.ed25519_privkey = colBlob(stmt, 7);
-        r.signature       = colBlob(stmt, 8);
-        r.lastReadMsgId   = sqlite3_column_int64(stmt, 9);
-        r.lastPosMsgId    = sqlite3_column_int64(stmt, 10);
-        r.createdAt       = sqlite3_column_int64(stmt, 11);
+        r.x25519_pubkey   = colBlob(stmt, 3);
+        r.x25519_privkey  = colBlob(stmt, 4);
+        r.ed25519_pubkey  = colBlob(stmt, 5);
+        r.ed25519_privkey = colBlob(stmt, 6);
+        r.signature       = colBlob(stmt, 7);
+        r.lastReadMsgId   = sqlite3_column_int64(stmt, 8);
+        r.lastPosMsgId    = sqlite3_column_int64(stmt, 9);
+        r.createdAt       = sqlite3_column_int64(stmt, 10);
         list.append(r);
     }
     sqlite3_finalize(stmt);