This is a feature that allows to fetch the app status via a celery
task which stores the result in the Redis database and notifies the API
of the change. The API sends a notification to connected clients via the
SSE mechanism.
Using a background task allows to avoid crashing the whole API if the
script call fails.
The by default the cache is refreshed every 30 minutes. This can be
changed by setting the `BAPI_APP_STATUS_UPDATE_INTERVAL_MIN` environment
variable.
refs #123
