PermissionError: [Errno 13] Permission denied: 'db'

I sometimes see this error but it doesn't seem to cause issues.

```
Traceback (most recent call last):
File "/home/libretranslate/LibreTranslate/env/bin/libretranslate", line 8, in
sys.exit(main())
File "/home/libretranslate/LibreTranslate/env/lib/python3.10/site-packages/libretranslate/main.py", line 192, in main
app = create_app(args)
File "/home/libretranslate/LibreTranslate/env/lib/python3.10/site-packages/libretranslate/app.py", line 1069, in create_app
Session(app)
File "/home/libretranslate/LibreTranslate/env/lib/python3.10/site-packages/flask_session/__init__.py", line 54, in __init__
self.init_app(app)
File "/home/libretranslate/LibreTranslate/env/lib/python3.10/site-packages/flask_session/__init__.py", line 61, in init_app
app.session_interface = self._get_interface(app)
File "/home/libretranslate/LibreTranslate/env/lib/python3.10/site-packages/flask_session/__init__.py", line 90, in _get_interface
session_interface = FileSystemSessionInterface(
File "/home/libretranslate/LibreTranslate/env/lib/python3.10/site-packages/flask_session/sessions.py", line 322, in __init__
self.cache = FileSystemCache(cache_dir, threshold=threshold, mode=mode)
File "/home/libretranslate/LibreTranslate/env/lib/python3.10/site-packages/cachelib/file.py", line 65, in __init__
os.makedirs(self._path)
File "/usr/lib/python3.10/os.py", line 215, in makedirs
makedirs(head, exist_ok=exist_ok)
File "/usr/lib/python3.10/os.py", line 225, in makedirs
mkdir(name, mode)
PermissionError: [Errno 13] Permission denied: 'db'
```

Start by reviewing libretranslate/main.py and libretranslate/app.py around app creation and the Session(app) call, then inspect the Flask-Session and cachelib file-session configuration shown in the traceback. Reproduce startup under the affected permissions; done means the application starts without the PermissionError for the db directory.

Written by the indexing model from the issue text.