feat(DATA-004): add underlying instrument selector

app/models/portfolio.py

@@ -109,6 +109,9 @@ class PortfolioConfig:
     fallback_source: str = "yfinance"
     refresh_interval: int = 5
 
+    # Underlying instrument selection
+    underlying: str = "GLD"
+
     # Alert settings
     volatility_spike: float = 0.25
     spot_drawdown: float = 7.5
@@ -223,6 +226,7 @@ class PortfolioConfig:
             "primary_source": self.primary_source,
             "fallback_source": self.fallback_source,
             "refresh_interval": self.refresh_interval,
+            "underlying": self.underlying,
             "volatility_spike": self.volatility_spike,
             "spot_drawdown": self.spot_drawdown,
             "email_alerts": self.email_alerts,
@@ -285,6 +289,7 @@ class PortfolioRepository:
         "primary_source",
         "fallback_source",
         "refresh_interval",
+        "underlying",  # optional with default "GLD"
         "volatility_spike",
         "spot_drawdown",
         "email_alerts",
@@ -344,10 +349,14 @@ class PortfolioRepository:
         upgraded = cls._upgrade_legacy_default_workspace(deserialized)
         return PortfolioConfig.from_dict(upgraded)
 
+    # Fields that must be present in persisted payloads
+    # (underlying is optional with default "GLD")
+    _REQUIRED_FIELDS = _PERSISTED_FIELDS - {"underlying"}
+
     @classmethod
     def _validate_portfolio_fields(cls, payload: dict[str, Any]) -> None:
         keys = set(payload.keys())
-        missing = sorted(cls._PERSISTED_FIELDS - keys)
+        missing = sorted(cls._REQUIRED_FIELDS - keys)
         unknown = sorted(keys - cls._PERSISTED_FIELDS)
         if missing or unknown:
             details: list[str] = []