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.

config.js

@@ -66,6 +66,8 @@ export const CONFIG = {
     NOTIFICATIONS_CACHE_REL: '.config/peardock/cache/notifications.json',
     /** Local backup history: ~/.config/peardock/cache/backups.json */
     BACKUPS_CACHE_REL: '.config/peardock/cache/backups.json',
+    /** Server alerts snapshot (synced from admin export): ~/.config/peardock/cache/alerts.json */
+    ALERTS_CACHE_REL: '.config/peardock/cache/alerts.json',
     /** localStorage key mirrored from settings cache */
     SETTINGS_KEY: 'peardock.settings.v1',
   },