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 Removed the now-superseded GOTIFY_URL/GOTIFY_TOKEN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>

README.md

@@ -41,6 +41,10 @@ All modules from the original plan are built:
   Supports self-signed certificates (common in homelab Proxmox setups).
 - **Integrations → Synology** — volume and disk health (read-only by
   design). Supports self-signed certificates.
+- **Settings** (admin-only) — notification channels (Gotify, ntfy, SMTP,
+  generic webhook) with per-channel test buttons, per-event toggles (DNS
+  record added/updated/deleted, daily secret-expiry reminder with a
+  configurable time/timezone), and DNS provider badge-color customization.
 
 All six integrations follow the same config-in-UI + encrypted-credentials
 pattern, added through **Integrations → Manage integrations**.