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/public/openapi.json

@@ -152,6 +152,11 @@
           "ldapVlessField": {
             "type": "string"
           },
+          "nodeDownThreshold": {
+            "maximum": 100,
+            "minimum": 1,
+            "type": "integer"
+          },
           "outboundDownThreshold": {
             "maximum": 100,
             "minimum": 1,
@@ -639,6 +644,7 @@
           "ldapUserAttr",
           "ldapUserFilter",
           "ldapVlessField",
+          "nodeDownThreshold",
           "outboundDownThreshold",
           "pageSize",
           "panelOutbound",
@@ -932,6 +938,11 @@
           "ldapVlessField": {
             "type": "string"
           },
+          "nodeDownThreshold": {
+            "maximum": 100,
+            "minimum": 1,
+            "type": "integer"
+          },
           "outboundDownThreshold": {
             "maximum": 100,
             "minimum": 1,
@@ -1427,6 +1438,7 @@
           "ldapUserAttr",
           "ldapUserFilter",
           "ldapVlessField",
+          "nodeDownThreshold",
           "outboundDownThreshold",
           "pageSize",
           "panelOutbound",