Document every notification this app sends in NOTIFICATIONS.md

Covers all 16 notification types (daily reminders, state-based health/
automation alerts, real-time DNS and integration-down alerts, and the
quiet-hours digest), what triggers each, and what does and doesn't
respect quiet hours and maintenance windows. Every trigger condition
and threshold was cross-checked against the actual scheduler/monitor
code, not just the settings labels.

Also fixes the "Daily reminder time" hint text in NotificationSettings,
which had gone stale — it listed only 4 of the 6 checks that actually
share that schedule (missing domain expiry and PBS verification).

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

web/src/pages/settings/NotificationSettings.tsx

@@ -633,7 +633,10 @@ export default function NotificationSettings() {
                         disabled={!dailyChecksEnabled}
                         onChange={(e) => setNotifications((n) => ({ ...n, secretCheckTime: e.target.value }))}
                       />
-                      <div className="form-hint">Shared by the secret, Tailscale key, Docker update, and Proxmox backup reminders above.</div>
+                      <div className="form-hint">
+                        Shared by the secret, Tailscale key, Docker update, Proxmox backup, Proxmox Backup Server, and
+                        domain expiry reminders above.
+                      </div>
                     </div>
                     <div className="col-6">
                       <label className="form-label">Timezone</label>