fix(security): demo sends nothing, hidden SMTP password, body limits (FLAW-33..38)

- Demo: senders for mail, Discord and webhooks refuse, no alarm ticker;
  test alarm, SMTP test, server settings, backups, account and session
  changes of the shared demo accounts answer 403; the session list only
  shows the own device. The UI shows the notices and disables the controls.
- SMTP password is never sent back (smtpPassSet), empty keeps it.
- ?token= on webhook ingest accepts only API keys.
- Request bodies capped at 2 MB (sync 8 MB, imports 101 MB).
- Deleting an account removes its server backups.
- Backup downloads, exports and import uploads get 30 minutes.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

frontend/src/index.css

@@ -499,3 +499,9 @@ body {
     padding-bottom: 6px !important;
   }
 }
+
+/* Read-only sections (e.g. server settings in the demo): everything inside looks switched off */
+fieldset:disabled :is(button, input, select, textarea) {
+  opacity: 0.5;
+  cursor: not-allowed;
+}