Add fallback_reference for deviation when insufficient data points

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.

_data/indicators.json

@@ -0,0 +1,12 @@
+{
+    "wti_brent_ratio": {
+        "display_name": "WTI/BRENT",
+        "type": "ratio",
+        "numerator": "xyz:CL",
+        "denominator": "xyz:BRENTOIL",
+        "changes": ["1h", "1d"],
+        "show_deviation": true,
+        "min_data_points": 100,
+        "fallback_reference": 0.96065
+    }
+}