Add a new scheduled task that syncs RetroAchievements progress for all
users with a RetroAchievements username.
Environment variables:
- `ENABLE_SCHEDULED_RETROACHIEVEMENTS_PROGRESS_SYNC`: Enable or disable
the task (default: `false`)
- `SCHEDULED_RETROACHIEVEMENTS_PROGRESS_SYNC_CRON`: Cron string to
schedule the task (default: "0 4 * * *" - daily at 4 AM)
Guarantee that cache is initialized during startup, and only once,
instead of every time a `MetadataHandler` object is instantiated.
Also, improve logic to determine `fixtures` paths.
For steps that need to run before the web application starts, such as
scheduling tasks, this new `startup.py` script is introduced.
This fixes a recently introduced issue where task scheduling was not
being triggered, because of it being included in the
`if __name__ == "__main__":` block, which is not executed when
the application is run by Gunicorn in production environments.
We do not include this logic as part of FastAPI's lifespan
implementation, as running multiple workers with Gunicorn would
cause this logic to be executed multiple times.
