Topup now only accepts VND — users must convert to get other currencies.
Convert uses a 0.5% bid/ask spread: buying USD costs more VND (ask),
selling USD back gives less VND (bid). Simulates real forex behavior.
Paper trading system with 5 commands (trade_topup, trade_buy,
trade_sell, trade_convert, trade_stats). Supports VN stocks via TCBS,
crypto via CoinGecko, forex via ER-API, and gold via PAX Gold proxy.
Per-user portfolio stored in KV with 60s price caching. 54 new tests.
