Refactor code structure for improved readability and maintainability

app/services/score_fusion.py

@@ -167,6 +167,7 @@ class _ExternalScore:
     available: bool
     is_ai: bool
     confidence: float
+    classifier_used: str = "unknown"
 
 
 def _parse_external_music_ai(data: Optional[dict]) -> Optional[_ExternalScore]:
@@ -210,6 +211,7 @@ def _parse_clap_result(
         available=True,
         is_ai=result.is_ai,
         confidence=result.confidence,
+        classifier_used=getattr(result, "classifier_used", "unknown"),
     )