fix: use selected data source for backtest historical prices

The backtest engine was always using fixture data (limited to 2024-01-02
through 2024-01-08) regardless of the data_source selection. The fix
fetches historical prices using the specified data source (Databento,
Yahoo Finance, or synthetic) and passes them directly to the engine.

app/services/backtesting/ui_service.py

@@ -323,15 +323,20 @@ class BacktestPageService:
             margin_call_ltv=normalized_inputs.margin_call_ltv,
         )
 
-        # CRITICAL: The backtest engine uses a fixture provider (synthetic_v1),
+        # Fetch historical prices using the specified data source
-        # regardless of the data_source used for price fetching.
+        history = self.get_historical_prices(normalized_symbol, start_date, end_date, data_source)
-        # We must use the fixture provider's ID for the scenario, not the data source's ID.
+        if not history:
-        # The data_source parameter only affects price data fetching, not the backtest engine.
+            raise ValueError("No historical prices found for scenario window")
+        if history[0].date != start_date:
+            raise ValueError(
+                "Scenario start date must match the first available historical close for entry-at-start backtests"
+            )
+
+        # Use the fixture provider's ID for the scenario (for pricing mode)
+        # The actual price data comes from the specified data_source
         provider_id = self.backtest_service.provider.provider_id
         pricing_mode = self.backtest_service.provider.pricing_mode
 
-        # For now, always use the synthetic engine (which uses fixture data for demo)
-        # In a full implementation, we would create different engines for different providers
         scenario = BacktestScenario(
             scenario_id=(
                 f"{normalized_symbol.lower()}-{start_date.isoformat()}-{end_date.isoformat()}-{template.slug}"
@@ -353,9 +358,14 @@ class BacktestPageService:
         if data_source == "databento":
             data_cost_usd = self.get_cost_estimate(normalized_symbol, start_date, end_date, data_source)
 
+        # Run the backtest engine directly with pre-fetched history
+        # This bypasses the fixture provider in BacktestService.run_scenario
+        template_result = self.backtest_service.engine.run_template(scenario, template, history)
+        run_result = BacktestRunResult(scenario_id=scenario.scenario_id, template_results=(template_result,))
+
         return BacktestPageRunResult(
             scenario=scenario,
-            run_result=self.backtest_service.run_scenario(scenario),
+            run_result=run_result,
             entry_spot=entry_spot,
             data_source=data_source,
             data_cost_usd=data_cost_usd,