Build the Settings module: notification channels, event toggles, DNS badge colors

The Settings page was a "coming soon" placeholder. Port Sloth Manager's
settings feature set: Gotify/ntfy/SMTP/webhook notification channels
(each with its own test-send button), per-event toggles (DNS record
added/updated/deleted, a daily secret-expiry digest with configurable
time/timezone), and per-provider DNS badge color customization.

Settings persist in the existing `settings` key/value table via a new
settingsStore service; a notify service fans a message out to every
enabled channel. DNS record add/update/delete now fire notifications,
and a node-schedule job re-arms itself whenever the notification
settings change. Removed the now-superseded GOTIFY_URL/GOTIFY_TOKEN
env vars in favor of in-app configuration.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

server/src/env.ts

@@ -11,10 +11,6 @@ export const env = {
     clientId: process.env.AUTHENTIK_CLIENT_ID ?? "",
     clientSecret: process.env.AUTHENTIK_CLIENT_SECRET ?? "",
   },
-  gotify: {
-    url: process.env.GOTIFY_URL ?? "",
-    token: process.env.GOTIFY_TOKEN ?? "",
-  },
   get authEnabled() {
     return Boolean(this.authentik.issuerUrl && this.authentik.clientId && this.authentik.clientSecret);
   },