fix: race condition in os.makedirs causing worker crashes

Replace check-then-create pattern with exist_ok=True to prevent
FileExistsError when multiple workers initialize simultaneously.

Affects:
- subscriptions.py: open_database()
- watch.py: save_decrypt_cache()
- local_playlist.py: add_to_playlist()
- util.py: fetch_url(), get_visitor_data()
- settings.py: initialization

Fixes Gunicorn worker startup failures in multi-worker deployments.

settings.py

@@ -453,8 +453,7 @@ else:
     print("Running in non-portable mode")
     settings_dir = os.path.expanduser(os.path.normpath("~/.yt-local"))
     data_dir = os.path.expanduser(os.path.normpath("~/.yt-local/data"))
-    if not os.path.exists(settings_dir):
-        os.makedirs(settings_dir)
+    os.makedirs(settings_dir, exist_ok=True)
 
 settings_file_path = os.path.join(settings_dir, 'settings.txt')