Fix broll scene composition imports and typing

backend/services/podcast/broll_service.py

@@ -17,6 +17,10 @@ from loguru import logger
 
 # Import chart generators directly
 from services.podcast.broll_composer import (
+    Insight,
+    SceneAssets,
+    dispatch_scene,
+    compose_video,
     make_bar_chart,
     make_horizontal_bar,
     make_line_trend,
@@ -217,7 +221,7 @@ class BrollService:
             logger.error(f"[BrollService] Failed to compose final video: {e}")
             raise
     
-    def cleanup(self, file_paths: List[str] = None):
+    def cleanup(self, file_paths: Optional[List[str]] = None):
         """
         Clean up temporary B-roll files.
         
@@ -250,4 +254,4 @@ def get_broll_service(output_dir: Optional[str] = None) -> BrollService:
     global _broll_service_instance
     if _broll_service_instance is None:
         _broll_service_instance = BrollService(output_dir=output_dir)
-    return _broll_service_instance
+    return _broll_service_instance