* add configurable ttl for active alert notifications
Notifications sent while an alert was active always used a TTL of 0, so the push service dropped them if the device wasn't reachable at that moment and users only received the notification sent when the alert ended. A new `ttl` notifications option, settable globally or per camera, lets the push service retry these notifications for that many seconds. It defaults to 0, and the settings UI warns that a non-zero value can deliver stale notifications.
* make notification ttl a global advanced setting
- Read `ttl` from the global notifications config and hide it on the camera form
- Show it under Advanced Settings on the Notifications page
- Remove extra padding that misaligned the Notifications cards with the rest of the form
- Shorten the ttl warning
* tweaks
* use enum instead of raw seconds value