db_sync: LMDB→SQLite, multi-instance, etcp_api, chatgui integration

- src/db_sync: LMDB replaced with SQLite (separate tables per instance: db_sync_name_id)
  TTL index (author, timestamp), WAL mode, per-instance peers/timers
  opaque DB_SYNC_INSTANCE* handle, insert callback, select API
  new fields: author_signature, delivered_peers, delivery_chain
  transport: etcp_api direct P2P (was etcp_router)

- src/db_sync.h: user-oriented API docs, multi-instance lifecycle

- lib/Makefile.am: liblmdb sources removed
- lib/sqlite3.c+h: SQLite amalgamation added

- topo_node: LMDB→SQLite (src/topo_node_sqlite.c moved from chatgui)

- chatgui: real db_sync.c replaces db_sync_stub.c
  chat_core: db_sync_insert_signed + dual-write insert callback
  utun_node: db_sync_init/destroy replaces chat_sync_init/destroy
  CMakeLists: stubs removed, real db_sync.c added

tools/chatgui/transport/utun_node.cpp

@@ -22,6 +22,7 @@ extern "C" {
 #include "../lib/mem.h"
 #include "chat_sync.h"
 #include "chat_core.h"
+#include "db_sync.h"
 #include "gui_bridge.h"
 }
 
@@ -237,10 +238,10 @@ void UtunNode::runLoop() {
 
     etcp_router_bind(m_instance, ETCP_RT_ID_CHAT, recvCallback);
 
-    /* Initialize chat_core (DB) and chat_sync */
+    /* Initialize chat_core (DB) and db_sync (message sync) */
     chat_core_init(m_instance, m_dbPath.toUtf8().constData());
-    chat_sync_init(m_instance, NULL);
-    DEBUG_INFO(DEBUG_CATEGORY_DEBUG, "chat_core + chat_sync initialized");
+    db_sync_init(m_instance);
+    DEBUG_INFO(DEBUG_CATEGORY_DEBUG, "chat_core + db_sync initialized");
     DEBUG_INFO(DEBUG_CATEGORY_DEBUG, "utun_node: entering poll loop");
 
     while (!m_stop) {
@@ -248,7 +249,7 @@ void UtunNode::runLoop() {
     }
 
     etcp_router_bind(m_instance, ETCP_RT_ID_CHAT, nullptr);
-    chat_sync_destroy(m_instance);
+    db_sync_destroy(m_instance);
     chat_core_destroy(m_instance);
     utun_instance_destroy(m_instance);
     m_instance = nullptr;