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/schemas/setting.ts

@@ -39,6 +39,7 @@ export const AllSettingSchema = z
     tgBotBackup: z.boolean().optional(),
     tgCpu: z.number().int().min(0).max(100).optional(),
     outboundDownThreshold: z.number().int().min(1).max(100).optional(),
+    nodeDownThreshold: z.number().int().min(1).max(100).optional(),
     tgLang: z.string().optional(),
     twoFactorEnable: z.boolean().optional(),
     twoFactorToken: z.string().optional(),