Change VAPID expiration from 24h to 12h

[collimarco]

The default value of 24h is not a safe default value.

Sometimes when you restart a server and the clock is wrong by a few seconds, it may result in notifications being rejected by Google Firebase.

Note that 24h is the limit for Firebase, so even a small bias can cause issues.

12h is a better default because it ensures the notification delivery even if the clock is not perfect.

12h is the average between 24h (max) and 0 (min) and, given a random deviation in the clock, it maximizes the probability of successful notifications.