[Feature] Server notifies clients when it becomes available

I have searched the existing feature requests, both open and closed, to make sure this is not a duplicate request. • Yes The feature Clients can only discover that the server is up by asking it. There is no way for the server to announce it. On a server that is not always running, the client's polling schedule and the server's availability have to coincide by chance. Running Immich on infrastructure that stops when idle and starts on demand is a common self-hosted setup, and it hits this consistently. With a server that sleeps: • The Android periodic worker runs hourly (PeriodicWorkRequestBuilder(1, HOURS, 15, MINUTES)). • Its first request wakes the server but meets a 503 while it is still starting, so _syncAssets fails and the run is abandoned before _handleBackup(). • An hour later the server is asleep again, and the next run meets the same 503. Backup never completes, rather than completing late. Proposal: a server-lifecycle event, fired wh…