Convert `MetadataHandler` to an abstract base class and add an
`is_enabled` class method that allows every metadata handler to
independently report whether it is enabled based on its configuration.
This avoids the need for global variables in the config module, allowing
us to change the enabled state of a metadata handler at runtime if
needed.
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)
