fix: correct type annotations in databento_source.py

- Fix return type of _load_from_cache and _df_to_daily_points to return list[DailyClosePoint]
- Import DailyClosePoint from historical_provider
- Use TYPE_CHECKING pattern for optional databento/pandas imports

app/services/backtesting/databento_source.py

@@ -8,20 +8,31 @@ import logging
 from dataclasses import dataclass
 from datetime import date, timedelta
 from pathlib import Path
-from typing import Any
+from typing import TYPE_CHECKING, Any
+
+from app.services.backtesting.historical_provider import DailyClosePoint
+
+if TYPE_CHECKING:
+    import databento as db
+    import pandas as pd
+else:
+    db = None
+    pd = None
+
+DATABENTO_AVAILABLE = False
 
 logger = logging.getLogger(__name__)
 
 # Try to import databento, gracefully degrade if not available
 try:
-    import databento as db
-    import pandas as pd
+    import databento as _db
+    import pandas as _pd
 
+    db = _db
+    pd = _pd
     DATABENTO_AVAILABLE = True
 except ImportError:
-    db = None
-    pd = None
-    DATABENTO_AVAILABLE = False
+    pass
 
 
 @dataclass
@@ -98,7 +109,7 @@ class DatabentoHistoricalPriceSource:
             self._client = db.Historical(key=self.config.api_key)
         return self._client
 
-    def _load_from_cache(self, key: DatabentoCacheKey) -> list[dict[str, Any]] | None:
+    def _load_from_cache(self, key: DatabentoCacheKey) -> list[DailyClosePoint] | None:
         """Load cached data if available and fresh."""
         cache_file = key.cache_path(self.config.cache_dir)
         meta_file = key.metadata_path(self.config.cache_dir)
@@ -166,7 +177,7 @@ class DatabentoHistoricalPriceSource:
         )
         return data.to_df()
 
-    def _df_to_daily_points(self, df: Any) -> list[Any]:
+    def _df_to_daily_points(self, df: Any) -> list[DailyClosePoint]:
         """Convert DataFrame to DailyClosePoint list with OHLC data."""
         from app.services.backtesting.historical_provider import DailyClosePoint