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/zod.ts

@@ -63,6 +63,7 @@ export const AllSettingSchema = z.object({
   ldapUserAttr: z.string(),
   ldapUserFilter: z.string(),
   ldapVlessField: z.string(),
+  nodeDownThreshold: z.number().int().min(1).max(100),
   outboundDownThreshold: z.number().int().min(1).max(100),
   pageSize: z.number().int().min(0).max(1000),
   panelOutbound: z.string(),
@@ -253,6 +254,7 @@ export const AllSettingViewSchema = z.object({
   ldapUserAttr: z.string(),
   ldapUserFilter: z.string(),
   ldapVlessField: z.string(),
+  nodeDownThreshold: z.number().int().min(1).max(100),
   outboundDownThreshold: z.number().int().min(1).max(100),
   pageSize: z.number().int().min(0).max(1000),
   panelOutbound: z.string(),