Replace hardcoded 9-symbol registry with dynamic TCBS-based resolution.
Any VN stock ticker is now resolved on first use and cached in KV
permanently. Portfolio flattened from 4 category maps to single assets
map with automatic migration of old format. Crypto, gold, and currency
exchange disabled with "coming soon" message.
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.
