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.
Add optional min_data_points (default 100) and fallback_reference config
fields to indicator definitions. When available daily data points are
below min_data_points, the fallback_reference value is used as the
deviation reference instead of the computed mean.
Applied to ratio, price, spread, and diff_pct indicator types.
Configured WTI/BRENT ratio with fallback_reference=0.96065.
