Fix send_notification background task

[beenje]

We shouldn't pass the session to the background task. get_db() dependency will close the session after the request. There is no warranty the session won't be closed before the background task is done.

Instead we create a local session in the background task.

Avoid:

File "/app/app/utils.py", line 82, in send_notification
    for user_notification in notification.users_notification:
                             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/venv/lib/python3.11/site-packages/sqlalchemy/orm/attributes.py", line 294, in __get__
    return self.impl.get(instance_state(instance), dict_)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/venv/lib/python3.11/site-packages/sqlalchemy/orm/attributes.py", line 730, in get
    value = self.callable_(state, passive)
            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/venv/lib/python3.11/site-packages/sqlalchemy/orm/strategies.py", line 717, in _load_for_state
    raise orm_exc.DetachedInstanceError(
sqlalchemy.orm.exc.DetachedInstanceError: Parent instance <Notification at 0x7f1f8fb00c50> is not bound to a Session; lazy load operation of attribute 'users_notification' cannot proceed (Background on this error at: http://sqlalche.me/e/13/bhk3)