Replace hardcoded 0.5% spread with live buy/sell rates from BIDV bank
API. Buying USD uses bank's sell rate (higher), selling USD uses bank's
buy rate (lower). Reply shows both rates and actual spread percentage.
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.