Add notifications for collector failures

We should add an API endpoint so the collector can report smartctl --scan and smartctl --info errors. The endpoint can trigger an opt-out notification. If the backend receives a smartctl --xall json object indicating an error (see #944), a notification should also be sent.

This needs a new handler, a new route:

https://github.com/AnalogJ/scrutiny/blob/9d1ce790d0cad95820f8148af272114f7f5c22a9/webapp/backend/pkg/web/server.go#L22-L53

and a new setting, including a UI element and database migration. See https://github.com/AnalogJ/scrutiny/commit/bdbe13e320673d017c9c5cf3aea23436c79642c4 for an example.