Add heartbeat events to SSE connections

[Snd-R]

Some proxies like Cloudflare can timeout SSE connection if no data is sent over a period of time Snd-R/Komelia#112

I propose to add a comment only heartbeat event. Events with just the comment data are usually skipped by SSE clients
https://developer.mozilla.org/en-US/docs/Web/API/Server-sent_events/Using_server-sent_events#event_stream_format

Example implementation in SseController

  @Scheduled(fixedRate = 15_000)
  fun heartbeat() {
    if (emitters.isNotEmpty()) {
      synchronized(emitters) {
        emitters.forEach { (emitter, _) ->
          try {
            emitter.send(SseEmitter.event().comment("heartbeat"))
          } catch (_: IOException) {
          }
        }
      }
    }
  }
  

[gotson]

the number of tasks is sent regularly, albeit to admin accounts only. Doesn't that suffice ?

[gotson]

re-reading that as well as the komelia issue, it does make sense

[gotson]

Added in 1.24.0