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/ar-EG.json

@@ -1488,6 +1488,7 @@
       "eventGroupNode": "العقد",
       "eventOutboundDown": "غير متصل",
       "eventOutboundUp": "متصل",
+      "eventDownThresholdHint": "فحوصات فاشلة متتالية",
       "eventXrayCrash": "تعطّل",
       "eventNodeDown": "غير متصلة",
       "eventNodeUp": "متصلة",

internal/web/translation/en-US.json

@@ -1610,6 +1610,7 @@
       "eventGroupNode": "Nodes",
       "eventOutboundDown": "Down",
       "eventOutboundUp": "Up",
+      "eventDownThresholdHint": "failed checks in a row",
       "eventXrayCrash": "Crash",
       "eventNodeDown": "Down",
       "eventNodeUp": "Up",

internal/web/translation/es-ES.json

@@ -1488,6 +1488,7 @@
       "eventGroupNode": "Nodos",
       "eventOutboundDown": "Caído",
       "eventOutboundUp": "Activo",
+      "eventDownThresholdHint": "comprobaciones fallidas seguidas",
       "eventXrayCrash": "Caída",
       "eventNodeDown": "Caído",
       "eventNodeUp": "Activo",

internal/web/translation/fa-IR.json

@@ -1492,6 +1492,7 @@
       "eventGroupNode": "نودها",
       "eventOutboundDown": "قطع",
       "eventOutboundUp": "وصل",
+      "eventDownThresholdHint": "بررسی ناموفق پیاپی",
       "eventXrayCrash": "کرش",
       "eventNodeDown": "قطع",
       "eventNodeUp": "وصل",

internal/web/translation/id-ID.json

@@ -1488,6 +1488,7 @@
       "eventGroupNode": "Node",
       "eventOutboundDown": "Mati",
       "eventOutboundUp": "Aktif",
+      "eventDownThresholdHint": "pemeriksaan gagal berturut-turut",
       "eventXrayCrash": "Crash",
       "eventNodeDown": "Mati",
       "eventNodeUp": "Aktif",

internal/web/translation/ja-JP.json

@@ -1488,6 +1488,7 @@
       "eventGroupNode": "ノード",
       "eventOutboundDown": "ダウン",
       "eventOutboundUp": "アップ",
+      "eventDownThresholdHint": "回連続でチェック失敗",
       "eventXrayCrash": "クラッシュ",
       "eventNodeDown": "ダウン",
       "eventNodeUp": "アップ",

internal/web/translation/pt-BR.json

@@ -1488,6 +1488,7 @@
       "eventGroupNode": "Nós",
       "eventOutboundDown": "Inativo",
       "eventOutboundUp": "Ativo",
+      "eventDownThresholdHint": "verificações com falha seguidas",
       "eventXrayCrash": "Falha",
       "eventNodeDown": "Inativo",
       "eventNodeUp": "Ativo",

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",

internal/web/translation/tr-TR.json

@@ -1488,6 +1488,7 @@
       "eventGroupNode": "Düğümler",
       "eventOutboundDown": "Çevrimdışı",
       "eventOutboundUp": "Çevrimiçi",
+      "eventDownThresholdHint": "art arda başarısız kontrol",
       "eventXrayCrash": "Çökme",
       "eventNodeDown": "Çevrimdışı",
       "eventNodeUp": "Çevrimiçi",

internal/web/translation/uk-UA.json

@@ -1488,6 +1488,7 @@
       "eventGroupNode": "Вузли",
       "eventOutboundDown": "Недоступне",
       "eventOutboundUp": "Доступне",
+      "eventDownThresholdHint": "невдалих перевірок поспіль",
       "eventXrayCrash": "Збій",
       "eventNodeDown": "Недоступний",
       "eventNodeUp": "Доступний",

internal/web/translation/vi-VN.json

@@ -1488,6 +1488,7 @@
       "eventGroupNode": "Node",
       "eventOutboundDown": "Ngừng hoạt động",
       "eventOutboundUp": "Hoạt động",
+      "eventDownThresholdHint": "lần kiểm tra thất bại liên tiếp",
       "eventXrayCrash": "Sự cố",
       "eventNodeDown": "Ngừng hoạt động",
       "eventNodeUp": "Hoạt động",

internal/web/translation/zh-CN.json

@@ -1488,6 +1488,7 @@
       "eventGroupNode": "节点",
       "eventOutboundDown": "断开",
       "eventOutboundUp": "恢复",
+      "eventDownThresholdHint": "次连续检查失败",
       "eventXrayCrash": "崩溃",
       "eventNodeDown": "离线",
       "eventNodeUp": "上线",

internal/web/translation/zh-TW.json

@@ -1488,6 +1488,7 @@
       "eventGroupNode": "節點",
       "eventOutboundDown": "中斷",
       "eventOutboundUp": "恢復",
+      "eventDownThresholdHint": "次連續檢查失敗",
       "eventXrayCrash": "當機",
       "eventNodeDown": "離線",
       "eventNodeUp": "上線",