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/generated/examples.ts

@@ -39,6 +39,7 @@ export const EXAMPLES: Record<string, unknown> = {
     "ldapUserAttr": "",
     "ldapUserFilter": "",
     "ldapVlessField": "",
+    "nodeDownThreshold": 1,
     "outboundDownThreshold": 1,
     "pageSize": 0,
     "panelOutbound": "",
@@ -227,6 +228,7 @@ export const EXAMPLES: Record<string, unknown> = {
     "ldapUserAttr": "",
     "ldapUserFilter": "",
     "ldapVlessField": "",
+    "nodeDownThreshold": 1,
     "outboundDownThreshold": 1,
     "pageSize": 0,
     "panelOutbound": "",