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.

frontend/src/models/setting.ts

@@ -174,6 +174,7 @@ export class AllSetting {
   smtpCpu = 80;
   smtpMemory = 80;
   outboundDownThreshold = 3;
+  nodeDownThreshold = 1;
   hasTgBotToken = false;
   hasTwoFactorToken = false;
   hasLdapPassword = false;
@@ -216,6 +217,10 @@ export class AllSetting {
     this.outboundDownThreshold = Number.isFinite(threshold)
       ? Math.min(100, Math.max(1, threshold))
       : 3;
+    const nodeThreshold = Math.round(Number(this.nodeDownThreshold));
+    this.nodeDownThreshold = Number.isFinite(nodeThreshold)
+      ? Math.min(100, Math.max(1, nodeThreshold))
+      : 1;
   }
 
   equals(other: AllSetting): boolean {