Add configurable notification TTL (#24829)

* 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

docs/docs/configuration/advanced/reference.md

@@ -578,6 +578,10 @@ notifications:
   email: "admin@example.com"
   # Optional: Cooldown time for notifications in seconds (default: shown below)
   cooldown: 0
+  # Optional: Delivery of notifications sent while an alert is active (default: shown below)
+  # Options are immediate (deliver right away or drop) and reliable (retry for up to 30 seconds)
+  # NOTE: reliable can deliver late notifications that still link to the live view
+  ttl: immediate
 
 # Optional: Record configuration
 # NOTE: Can be overridden at the camera level