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 Removed the now-superseded GOTIFY_URL/GOTIFY_TOKEN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>

.env.example

@@ -23,6 +23,5 @@ AUTHENTIK_ISSUER_URL=https://authentik.example.lan/application/o/homelab-manager
 AUTHENTIK_CLIENT_ID=
 AUTHENTIK_CLIENT_SECRET=
 
-# --- Optional: Gotify notifications (secret expiry, integration offline, etc.) ---
-GOTIFY_URL=
-GOTIFY_TOKEN=
+# Notification channels (Gotify, ntfy, SMTP, webhook) are configured in-app
+# under Settings, not here.