fix credentials error

app.py

@@ -4,14 +4,12 @@ import pysrt
 import os
 from pydub import AudioSegment
 from dotenv import load_dotenv
+from google.oauth2 import service_account
+import json
 
 # 加载环境变量
 load_dotenv()
 
-import os
-from google.oauth2 import service_account
-from google.cloud import texttospeech
-
 def get_google_client():
     # Get credentials from environment variable
     creds_json = os.getenv("GOOGLE_CREDENTIALS_JSON")