fix: add debug logging for vocal extraction failures in extract_sample_features

app/training/extract_features_batch.py

@@ -108,7 +108,8 @@ def extract_sample_features(audio_path: str) -> dict | None:
                 "vocal_harmonic_ratio": vocals.vocal_harmonic_ratio,
                 "vocal_energy_ratio": vocals.vocal_energy_ratio,
             })
-        except Exception:
+        except Exception as e:  # noqa: BLE001
+            logger.debug("Vocal extraction failed: %s", e)
             # Fill vocal features with defaults
             row.update({
                 "has_vocals": 0.0,