fix: convert PERMANENT_SESSION_LIFETIME to int

[jackylamhk]

Flask passes the PERMANENT_SESSION_LIFETIME value directly to timedelta, which needs to be an integer.

Traceback (most recent call last):
  File "/mlflow/.venv/lib/python3.12/site-packages/flask/app.py", line 941, in finalize_request
    response = self.process_response(response)
               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/mlflow/.venv/lib/python3.12/site-packages/flask/app.py", line 1322, in process_response
    self.session_interface.save_session(self, ctx.session, response)
  File "/mlflow/.venv/lib/python3.12/site-packages/flask_session/base.py", line 305, in save_session
    self._upsert_session(app.permanent_session_lifetime, session, store_id)
                         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/mlflow/.venv/lib/python3.12/site-packages/flask/config.py", line 42, in __get__
    rv = self.get_converter(rv)
         ^^^^^^^^^^^^^^^^^^^^^^
  File "/mlflow/.venv/lib/python3.12/site-packages/flask/sansio/app.py", line 56, in _make_timedelta
    return timedelta(seconds=value)
           ^^^^^^^^^^^^^^^^^^^^^^^^
TypeError: unsupported type for timedelta seconds component: str