Add experimental server alerts and webhook notifications

Server-side alerting for Docker/container/stack health with Discord,
Slack, Teams, ntfy, Gotify, Telegram, and generic webhooks. Settings
tab, client cache, and backup/restore coverage; marked experimental.

server/server.js

@@ -25,6 +25,7 @@ import {
   restoreTunnelsFromDisk,
 } from './services/holesail-tunnels.js'
 import { restoreSchedules } from './services/schedules.js'
+import { startAlerts, stopAlerts } from './services/alerts.js'
 import { docker } from './services/docker.js'
 import logger from './utils/logger.js'
 import { isSwarmEnabled } from './handlers/swarm.js'
@@ -157,6 +158,11 @@ if (!dockerStatus.ok) {
 
 startDockerEventStream()
 startStatsBroadcast()
+try {
+  startAlerts()
+} catch (err) {
+  log.warn('Alerts engine failed to start', { error: err.message })
+}
 
 if (isHolesailEnabled()) {
   restoreTunnelsFromDisk()
@@ -186,6 +192,11 @@ async function shutdown(signal = 'shutdown') {
 
   stopStatsBroadcast()
   stopDockerEventStream()
+  try {
+    stopAlerts()
+  } catch {
+    // ignore
+  }
 
   try {
     await closeAllTunnels()