Bypass SDK's name_to_coin lookup in candles_snapshot by calling
http_info.post directly with the raw coin name. This fixes
KeyError for symbols like xyz:GOLD, xyz:CL, mkts:USTECH that
aren't in the name_to_coin dictionary.
The WebSocket subscription already bypassed this lookup (line 154),
but the historical fetch path did not.
Add gold_silver_ratio indicator (xyz:GOLD / xyz:SILVER) with fallback
reference of 61.59, mirroring the existing WTI/BRENT ratio setup.
Also register xyz:GOLD and xyz:SILVER in WATCHED_COINS and data_fetcher
defaults so the candle data is fetched for the new indicator.