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

src/config_parser.h

@@ -114,7 +114,7 @@ struct global_config {
 
     // Debug and logging configuration
     char log_file[256];        // Path to log file (empty = stdout)
-    char db_path[256];         // Path to LMDB nodeinfo database (empty = disabled)
+    char db_path[256];         // Path to SQLite database (empty = disabled)
     int  db_sync_enabled;      // 1 = enable distributed DB sync (default: 0)
     uint32_t db_sync_ttl;      // TTL for unsent records in seconds (default: 86400)
     char debug_level[16];      // debug level: error, warn, info, debug, trace