Sync host & service check outputs to Notifications periodically

[yhabteab]

In order to always have the latest check output of a checkable, there is now a new goroutine that periodically (every 5 minutes) syncs the check output of all checkables that have an active incident state from the icinga:{host,service}:state Redis hashes to Icinga Notifications. This is necessary because a checkable's check can, for instance, be in CRITICAL state for a long time, while its output can change with each check result. And since that won't trigger any state change events, thus, not queued to the runtime updates stream, there's no way for Icinga Notifications to know about the updated output.

icingadb-1  | 2026-07-21T13:08:45.586+0200	DEBUG	notifications	Syncing check outputs for incidents	{"host_incidents": 0, "service_incidents": 10, "last_sync": "2026-07-21T13:03:45.586+0200"}
icingadb-1  | 2026-07-21T13:13:45.650+0200	DEBUG	notifications	Syncing check outputs for incidents {"host_incidents": 0, "service_incidents": 10, "last_sync": "2026-07-21T13:08:45.645+0200"}
icingadb-1  | 2026-07-21T13:18:45.719+0200	DEBUG	notifications	Syncing check outputs for incidents {"host_incidents": 0, "service_incidents": 10, "last_sync": "2026-07-21T13:13:45.718+0200"}
icingadb-1  | 2026-07-21T13:23:45.792+0200	DEBUG	notifications	Syncing check outputs for incidents	{"host_incidents": 0, "service_incidents": 10, "last_sync": "2026-07-21T13:18:45.792+0200"}
icingadb-1  | 2026-07-21T13:28:45.869+0200	DEBUG	notifications	Syncing check outputs for incidents	{"host_incidents": 0, "service_incidents": 10, "last_sync": "2026-07-21T13:23:45.869+0200"}

resolves #1140