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/rpc/register.js

@@ -20,6 +20,7 @@ import { registerRegistryHandlers } from '../handlers/registry.js'
 import { registerBinaryStreamHandlers } from './binary-stream.js'
 import { registerSuggestionHandlers } from '../handlers/suggestions.js'
 import { registerTunnelHandlers } from '../handlers/tunnels.js'
+import { registerAlertHandlers } from '../handlers/alerts.js'
 
 /**
  * @param {import('./session.js').PeerSession} session
@@ -27,6 +28,7 @@ import { registerTunnelHandlers } from '../handlers/tunnels.js'
 export function registerAllHandlers(session) {
   registerHandshake(session)
   registerSystemHandlers(session)
+  registerAlertHandlers(session)
   registerContainerHandlers(session)
   registerImageHandlers(session)
   registerNetworkHandlers(session)