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.

tests/unit/test_engine_faster_whisper.py

@@ -16,17 +16,18 @@ from luke_scribe.engine.faster_whisper_engine import FasterWhisperEngine
 
 
 class FakeWhisperModel:
-    """faster_whisper.WhisperModel 대체 — 생성 인자/세그먼트를 기록한다."""
+    """faster_whisper.WhisperModel 대체 — 생성 인자/세그먼트/info를 기록한다."""
 
     calls: list[dict] = []
     segments: list = []  # transcribe()가 yield할 세그먼트 (기본: namedtuple)
+    info: object = None  # 기본: TranscriptionInfo(namedtuple) 흉내
 
     def __init__(self, *args, **kwargs) -> None:
         self.kwargs = kwargs
         self.__class__.calls.append(kwargs)
 
     def transcribe(self, audio_path, **kwargs):
-        return iter(list(self.__class__.segments)), {}
+        return iter(list(self.__class__.segments)), self.__class__.info
 
 
 def _install_fake(monkeypatch) -> None:
@@ -35,6 +36,7 @@ def _install_fake(monkeypatch) -> None:
     monkeypatch.setitem(sys.modules, "faster_whisper", mod)
     FakeWhisperModel.calls.clear()
     FakeWhisperModel.segments = []
+    FakeWhisperModel.info = None
 
 
 def _opts(**kw) -> TranscriptionOptions:
@@ -137,3 +139,33 @@ def test_dict_segments_passthrough(monkeypatch):
     )
     segs = list(outcome.segments)
     assert segs == [{"index": 0, "start": 0.0, "end": 1.0, "text": "x"}]
+
+
+def test_namedtuple_info_normalized_to_dict(monkeypatch):
+    """faster-whisper TranscriptionInfo(namedtuple) → dict (GPU 실전 버그)."""
+    from collections import namedtuple
+
+    _install_fake(monkeypatch)
+    Info = namedtuple(
+        "TranscriptionInfo",
+        ["language", "language_probability", "duration", "duration_after_vad"],
+    )
+    FakeWhisperModel.info = Info(
+        language="ko", language_probability=0.99, duration=10.464, duration_after_vad=9.088
+    )
+    outcome = FasterWhisperEngine().transcribe(
+        "/tmp/x.wav", _opts(device="cpu", compute_type="int8")
+    )
+    # dict 계약: .get() 사용 가능 (batch.py가 이걸로 접근)
+    assert outcome.info.get("language") == "ko"
+    assert outcome.info.get("duration") == 10.464
+
+
+def test_dict_info_passthrough(monkeypatch):
+    """이미 dict인 info는 그대로 (mock 계약과 호환)."""
+    _install_fake(monkeypatch)
+    FakeWhisperModel.info = {"language": "ko"}
+    outcome = FasterWhisperEngine().transcribe(
+        "/tmp/x.wav", _opts(device="cpu", compute_type="int8")
+    )
+    assert outcome.info == {"language": "ko"}