chat: add ll_queue with index for P2P connections (replaces linear search)

- utun_instance.h: add chat_connections queue (indexed by node_id, 8 bytes)
- chat_sync.c: queue_new in init, queue_free in destroy, queue_find_data_by_index in cs_find_conn_for_node
- chat_sync.c: remove from queue in cs_on_conn_down
- chat_core.c: add to queue in connect_from_invite and connect_auto
- chat_core.c: remove from queue in cc_parallel_cleanup and ca_cleanup
- db_sync.c: replace linear conn search with queue_find_data_by_index (db_sync_send_hash)

src/utun_instance.h

@@ -135,6 +135,7 @@ struct UTUN_INSTANCE {
     // etcp_router bindings и seq-connections (per-instance service routing)
     struct ETCP_ROUTER_BINDINGS router_bindings;
     struct ll_queue* router_conns;
+    struct ll_queue* chat_connections; // chat P2P connections indexed by node_id
     struct CONN_MGR* conn_mgr;       // Connection Manager (может быть NULL)
     struct DB_SYNC* db_sync;         // Distributed DB sync (может быть NULL)