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/lib/types.ts

@@ -180,7 +180,8 @@ export interface SystemSettings {
   smtpHost?: string;
   smtpPort?: string; // sent as text, like the server stores it
   smtpUser?: string;
-  smtpPass?: string;
+  smtpPass?: string; // write only: the server never sends it back, empty keeps the stored one
+  smtpPassSet?: boolean;
   smtpFrom?: string;
   smtpTls?: boolean;
   smtpSkipVerify?: boolean;