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

lib/Makefile.am

@@ -29,18 +29,16 @@ libuasync_a_SOURCES = \
 	serialize.h \
 	tcp_io.c \
 	tcp_io.h \
-	liblmdb/mdb.c \
-	liblmdb/midl.c \
-	liblmdb/lmdb.h \
-	liblmdb/midl.h
+	sqlite3.c \
+	sqlite3.h
 
 libuasync_a_CFLAGS = \
 	-D_ISOC99_SOURCE \
 	-DDEBUG_OUTPUT_STDERR \
+	-DSQLITE_THREADSAFE=1 \
 	-g \
 	-I$(top_srcdir)/src \
-	-I$(top_srcdir)/lib \
-	-I$(srcdir)/liblmdb
+	-I$(top_srcdir)/lib
 
 # Clean build directory
 clean-local: