fix: normalize faster-whisper TranscriptionInfo to dict

Colab run 4 (A100): same namedtuple bug as segments — faster-whisper
returns TranscriptionInfo (namedtuple) but batch.py reads
outcome['info'].get('language') -> AttributeError 'TranscriptionInfo'
object has no attribute 'get' on every real transcription, failing the
CLI, API auto-worker job, worker drain, and bench clip alike.

FasterWhisperEngine now normalizes info to a dict at the boundary
(_to_dict_info: _asdict -> dataclasses.asdict -> known-field fallback).

+ 2 unit tests (namedtuple/dict info); 138 tests pass, ruff clean.

src/luke_scribe/engine/faster_whisper_engine.py

@@ -133,7 +133,7 @@ class FasterWhisperEngine(TranscriptionEngine):
         wrapped = _CancellableSegmentIterator(
             self._to_dict_segments(segments_iter), should_cancel or (lambda: False)
         )
-        return TranscriptionOutcome(wrapped, info=info)
+        return TranscriptionOutcome(wrapped, info=self._to_dict_info(info))
 
     @staticmethod
     def _to_dict_segments(segments):
@@ -169,6 +169,32 @@ class FasterWhisperEngine(TranscriptionEngine):
                 if (v := getattr(seg, k, None)) is not None
             }
 
+    @staticmethod
+    def _to_dict_info(info):
+        """faster-whisper TranscriptionInfo(namedtuple) → dict.
+
+        다운스트림(배치 파이프라인)은 info를 dict 계약으로 접근한다
+        (``info.get("language")``) — GPU 실전에서만 재현되는 버그:
+        'TranscriptionInfo' object has no attribute 'get'.
+        """
+        if info is None or isinstance(info, dict):
+            return info
+        asdict = getattr(info, "_asdict", None)
+        if asdict is not None:
+            return dict(asdict())
+        try:
+            import dataclasses
+
+            if dataclasses.is_dataclass(info):
+                return dataclasses.asdict(info)
+        except Exception:
+            pass
+        return {
+            k: v
+            for k in ("language", "language_probability", "duration", "duration_after_vad")
+            if (v := getattr(info, k, None)) is not None
+        }
+
     @staticmethod
     def _split_device(device: str) -> tuple[str, int]:
         """'cuda:N' → ('cuda', N). CTranslate2는 device='cuda'만 허용하므로 분리한다.