fix: restore clear_cache and get_cache_stats methods in DatabentoHistoricalPriceSource

- Add back clear_cache method that was accidentally removed
- Add file_count and total_size_bytes to get_cache_stats return value
- Update tests for fixed March 2026 default dates

app/services/backtesting/databento_source.py

@@ -318,9 +318,12 @@ class DatabentoHistoricalPriceSource:
         """Get cache statistics."""
         cache_dir = self.config.cache_dir
         if not cache_dir.exists():
-            return {"status": "empty", "entries": []}
+            return {"status": "empty", "entries": [], "file_count": 0, "total_size_bytes": 0}
 
         entries = []
+        total_size = 0
+        file_count = 0
+
         for meta_file in cache_dir.glob("*_meta.json"):
             try:
                 with open(meta_file) as f:
@@ -336,7 +339,39 @@ class DatabentoHistoricalPriceSource:
                         "cost_usd": meta.get("cost_usd"),
                     }
                 )
+                total_size += meta_file.stat().st_size
+                file_count += 1
             except Exception:
                 continue
 
-        return {"status": "populated" if entries else "empty", "entries": entries}
+        # Count parquet files too
+        for parquet_file in cache_dir.glob("dbn_*.parquet"):
+            total_size += parquet_file.stat().st_size
+            file_count += 1
+
+        return {
+            "status": "populated" if entries else "empty",
+            "entries": entries,
+            "file_count": file_count,
+            "total_size_bytes": total_size,
+        }
+
+    def clear_cache(self) -> int:
+        """Clear all cache files.
+
+        Returns:
+            Number of files deleted.
+        """
+        cache_dir = self.config.cache_dir
+        if not cache_dir.exists():
+            return 0
+
+        count = 0
+        for cache_file in cache_dir.glob("dbn_*.parquet"):
+            cache_file.unlink()
+            count += 1
+        for meta_file in cache_dir.glob("dbn_*_meta.json"):
+            meta_file.unlink()
+            count += 1
+
+        return count