feat(notifications): add a failed-check threshold for node.down alerts (#6768)

One lost heartbeat on a lossy link flipped a node offline and sent node.down,
and the next probe sent node.up, so a flaky node flooded Telegram, email and
Discord with alert pairs.

A new nodeDownThreshold setting (default 1, today's behaviour) works like
outboundDownThreshold: node.down fires once the failed-check streak reaches it,
once per outage, and node.up only after a node.down was sent. The announcement
is persisted in nodes.down_notified, so a restart inside the threshold window
neither drops the outage's node.down nor re-announces nodes already reported
offline; nodes offline at upgrade are seeded as announced.

internal/web/translation/ru-RU.json

@@ -1488,6 +1488,7 @@
       "eventGroupNode": "Узлы",
       "eventOutboundDown": "Недоступен",
       "eventOutboundUp": "Работает",
+      "eventDownThresholdHint": "неудачных проверок подряд",
       "eventXrayCrash": "Сбой",
       "eventNodeDown": "Недоступен",
       "eventNodeUp": "В сети",