Add GOLD/SILVER ratio indicator to dashboard

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.

WIKI/indicators.md

@@ -73,6 +73,19 @@ Computes `numerator / denominator`.
 - **1D Change**: compares to ratio from 1d candle close prices
 - **Deviation**: `(current - long_avg) / long_avg * 100`, where `long_avg` is the mean of daily ratios over all available history. If fewer than `min_data_points` (default 100) daily data points exist and `fallback_reference` is set, the fallback value is used instead.
 
+```json
+"gold_silver_ratio": {
+    "display_name": "GOLD/SILVER",
+    "type": "ratio",
+    "numerator": "xyz:GOLD",
+    "denominator": "xyz:SILVER",
+    "changes": ["1h", "1d"],
+    "show_deviation": true,
+    "min_data_points": 100,
+    "fallback_reference": 61.59
+}
+```
+
 ### `price` — Single Price
 
 ```json