* Add support for alerts via shoutrrr
This provides users the ability to use a wide variety of notification platforms instead of just email. If there's a problem sending a notification via shoutrrr, an error is logged and email is attempted as a fallback.
Since this uses Viper, users can set a notification type and URL via either a config file or environment variable. In the beszel_data folder (where the sqlite dbs reside), create an alerts.env file to set values that way.
Values:
* NOTIFICATION_TYPE
* If this is `shoutrrr`, then the shoutrrr library is used. Any other value, including not being set, uses the fallback email behavior.
* NOTIFICATION_URL
* If NOTIFICATION_TYPE is shoutrrr, this is the URL given to shoutrrr to send the alert. See list of supported services: https://containrrr.dev/shoutrrr/services/overview/
Note: there's currently a bug in viper v1.18.2+ where environment variable overrides aren't functioning when no config file exists, so this library should remain pinned to 1.18.1 until that's fixed. See: https://github.com/spf13/viper/issues/1895
* Update documentation
* Log shoutrrr URL instead of unused "to" var
