Add tickstem/heartbeat to Scheduler section

Author: mikhail.barsukou

README.md

@@ -1579,6 +1579,7 @@ _Libraries for scheduling jobs._
 - [scheduler](https://github.com/yuseferi/scheduler) - Go-native distributed job scheduler with delayed tasks, batched Redis coordination, retries, lease-based recovery, and versioned queue partitioning.
 - [tasks](https://github.com/madflojo/tasks) - An easy to use in-process scheduler for recurring tasks in Go.
 - [tickstem/cron](https://github.com/tickstem/cron) - Go client for scheduling HTTP cron jobs, with execution history, failure alerts, and tsk-local for testing handlers without live credentials.
+- [tickstem/heartbeat](https://github.com/tickstem/heartbeat) - Go client for dead-man's switch heartbeat monitoring: ping a URL after each job run and get alerted by email if pings stop arriving.
 
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