fix: split 'cuda:N' into device+device_index for CTranslate2

Colab A100 run exposed a GPU-only bug: DeviceManager returns
selected_device='cuda:0' and the engine passed it verbatim to
faster-whisper, but CTranslate2 only accepts device='cuda' with a
separate device_index arg -> 'unsupported device cuda:0' on every GPU
transcription. Fix: FasterWhisperEngine._split_device() splits
'cuda:N' -> ('cuda', N) and passes device_index to WhisperModel.

Notebook: server cell now pkills stale servers (old process holds
port 8000 and 401s on new keys since KeyStore loads at startup),
sets LUKESCRIBE_API_KEY_FILE explicitly, and verifies auth with
RAW_KEY before proceeding; worker cell guards read_result() is None.

+ 4 unit tests (device split contract), 131 tests pass, ruff clean.

src/luke_scribe/engine/faster_whisper_engine.py

@@ -86,10 +86,15 @@ class FasterWhisperEngine(TranscriptionEngine):
         with self._lock:
             model = self._models.get(model_key)
             if model is None:
+                # CTranslate2는 device 문자열로 "cuda"만 허용한다 ("cuda:0" 형태는
+                # "unsupported device cuda:0" 오류). DeviceManager가 내려주는
+                # "cuda:N"을 device + device_index로 분리해 전달한다.
+                device_name, device_index = self._split_device(options.device)
                 try:
                     model = WhisperModel(
                         options.model,
-                        device=options.device,
+                        device=device_name,
+                        device_index=device_index,
                         compute_type=options.compute_type,
                         download_root=self._download_root(),
                     )
@@ -128,6 +133,20 @@ class FasterWhisperEngine(TranscriptionEngine):
         wrapped = _CancellableSegmentIterator(segments_iter, should_cancel or (lambda: False))
         return TranscriptionOutcome(wrapped, info=info)
 
+    @staticmethod
+    def _split_device(device: str) -> tuple[str, int]:
+        """'cuda:N' → ('cuda', N). CTranslate2는 device='cuda'만 허용하므로 분리한다.
+
+        'cpu'/'cuda' 같은 인덱스 없는 값은 그대로 (device, 0)을 반환한다.
+        """
+        if device.startswith("cuda") and ":" in device:
+            name, _, idx = device.partition(":")
+            try:
+                return name, int(idx)
+            except ValueError:
+                pass
+        return device, 0
+
     def _download_root(self) -> str | None:
         from ..config import get_settings