fix: 修正 transcript 相關變數初始化與錯誤訊息，避免 referenced before assignment 問題

app.py

@@ -576,6 +576,7 @@ def convert_transcription_to_json(original_transcription):
     return all_results
 
 def generate_transcription_by_whisper(video_id):
+    print("====generate_transcription_by_whisper====")
     youtube_url = f'https://www.youtube.com/watch?v={video_id}'
     codec_name = "mp3"
     outtmpl = f"{OUTPUT_PATH}/{video_id}.%(ext)s"