- Replace db_sync_stub.c with real src/db_sync.c in libutun
- Remove message sync protocol from chat_sync (INIT_SYNC/INIT_RESP/SEND_DATA/PUSH/ACK_PUSH/SYNC_DONE)
- Keep chat_sync auxiliary: join protocol, peer management, auto-connect
- Remove chat_core_insert_record, cursor_*, mark_sent/ttl_delete stubs
- chat_core on_db_sync_insert now only does SQLite INSERT + GUI notify
- Move db_sync_init() from instance_init_common to utun_instance_init for config override timing
- Force db_sync_enabled=1 in utun_node.cpp
- db_sync handles all P2P message sync via service 0x20
Evgeny
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