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/components/ui/notifications/EmailNotifications.tsx

@@ -20,6 +20,7 @@ const GROUPS: NotificationGroupConfig[] = [
         key: 'outbound.down',
         label: 'eventOutboundDown',
         settingKey: 'outboundDownThreshold',
+        hint: 'eventDownThresholdHint',
         extra: ({ value, onChange, ariaLabel }) => (
           <InputNumber
             size="small"
@@ -44,7 +45,23 @@ const GROUPS: NotificationGroupConfig[] = [
     icon: <DesktopOutlined aria-hidden />,
     title: 'eventGroupNode',
     events: [
-      { key: 'node.down', label: 'eventNodeDown', settingKey: '' },
+      {
+        key: 'node.down',
+        label: 'eventNodeDown',
+        settingKey: 'nodeDownThreshold',
+        hint: 'eventDownThresholdHint',
+        extra: ({ value, onChange, ariaLabel }) => (
+          <InputNumber
+            size="small"
+            min={1}
+            max={100}
+            value={value}
+            onChange={onChange}
+            aria-label={ariaLabel}
+            style={{ width: 80 }}
+          />
+        ),
+      },
       { key: 'node.up', label: 'eventNodeUp', settingKey: '' },
     ],
   },

frontend/src/components/ui/notifications/NotificationGroup.tsx

@@ -1,4 +1,4 @@
-import { Space } from 'antd';
+import { Space, Typography } from 'antd';
 import { useTranslation } from 'react-i18next';
 import type { AllSetting } from '@/models/setting';
 import type { NotificationGroupConfig } from './types';
@@ -55,12 +55,22 @@ export function NotificationGroup({
             checked={selected.includes(event.key)}
             onToggle={() => onToggle(event.key)}
           >
-            {event.extra?.({
-              value:
-                Number((allSetting as unknown as Record<string, unknown>)[event.settingKey]) || 0,
-              onChange: (v) => updateSetting({ [event.settingKey]: v }),
-              ariaLabel: t(`pages.settings.${event.label}`),
-            })}
+            {event.extra && (
+              <Space size={8}>
+                {event.extra({
+                  value:
+                    Number((allSetting as unknown as Record<string, unknown>)[event.settingKey]) ||
+                    0,
+                  onChange: (v) => updateSetting({ [event.settingKey]: v }),
+                  ariaLabel: t(`pages.settings.${event.label}`),
+                })}
+                {event.hint && (
+                  <Typography.Text type="secondary">
+                    {t(`pages.settings.${event.hint}`)}
+                  </Typography.Text>
+                )}
+              </Space>
+            )}
           </NotificationEvent>
         ))}
       </Space>

frontend/src/components/ui/notifications/TelegramNotifications.tsx

@@ -20,6 +20,7 @@ const GROUPS: NotificationGroupConfig[] = [
         key: 'outbound.down',
         label: 'eventOutboundDown',
         settingKey: 'outboundDownThreshold',
+        hint: 'eventDownThresholdHint',
         extra: ({ value, onChange, ariaLabel }) => (
           <InputNumber
             size="small"
@@ -44,7 +45,23 @@ const GROUPS: NotificationGroupConfig[] = [
     icon: <DesktopOutlined aria-hidden />,
     title: 'eventGroupNode',
     events: [
-      { key: 'node.down', label: 'eventNodeDown', settingKey: '' },
+      {
+        key: 'node.down',
+        label: 'eventNodeDown',
+        settingKey: 'nodeDownThreshold',
+        hint: 'eventDownThresholdHint',
+        extra: ({ value, onChange, ariaLabel }) => (
+          <InputNumber
+            size="small"
+            min={1}
+            max={100}
+            value={value}
+            onChange={onChange}
+            aria-label={ariaLabel}
+            style={{ width: 80 }}
+          />
+        ),
+      },
       { key: 'node.up', label: 'eventNodeUp', settingKey: '' },
     ],
   },

frontend/src/components/ui/notifications/types.ts

@@ -4,6 +4,7 @@ export interface NotificationEventConfig {
   key: string;
   label: string;
   settingKey: string;
+  hint?: string;
   extra?: (props: {
     value: number;
     onChange: (v: number | null) => void;