[Feature] Support UnifiedPush (ntfy, Google's FCM, etc) for push notifications

I have searched the existing feature requests to make sure this is not a duplicate request. • Yes The feature This is the one missing link, I believe, that'd take Immich from a good app for shared albums, to a great app for shared albums. If spec is implemented, people would be able to use their own self-hosted ntfy instances, or some other method of their own choosing, to receive push notifications in a self-hosted fashion. I'm pretty sure the devs already know of it, but for others, here's what I'm talking about (from the FAQ section of the above website): ![image]( • Immich Server will send notifications to your self-hosted, or some other publicly hosted, push server. • Your push notification application will maintain an open connection to that public server at all times. • On getting a notification, your push application wakes up the Immich application on your phone to process the notification. Platform • Server…