fix(backtest): ensure data_source is passed through all validation calls

- Pass data_source to derive_entry_spot in backtests.py
- Remove default 'synthetic' value for data_source in derive_entry_spot and validate_preview_inputs
- Update all tests to explicitly pass data_source parameter
- Improve error message with helpful suggestion for Databento/Yahoo Finance

app/services/backtesting/ui_service.py

@@ -230,7 +230,7 @@ class BacktestPageService:
             for template in self.template_service.list_active_templates(symbol)
         ]
 
-    def derive_entry_spot(self, symbol: str, start_date: date, end_date: date, data_source: str = "synthetic") -> float:
+    def derive_entry_spot(self, symbol: str, start_date: date, end_date: date, data_source: str) -> float:
         history = self.get_historical_prices(symbol, start_date, end_date, data_source)
         if not history:
             raise ValueError("No historical prices found for scenario window")
@@ -251,7 +251,7 @@ class BacktestPageService:
         loan_amount: float,
         margin_call_ltv: float,
         entry_spot: float | None = None,
-        data_source: str = "synthetic",
+        data_source: str,
     ) -> float:
         normalized_symbol = symbol.strip().upper()
         if not normalized_symbol: