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>

internal/api/backups.go

@@ -162,7 +162,21 @@ func serveBackupFile(w http.ResponseWriter, r *http.Request, dir, name, contentT
 		writeError(w, http.StatusNotFound, err.Error())
 		return
 	}
+	allowLongTransfer(w)
 	w.Header().Set("Content-Type", contentType)
 	w.Header().Set("Content-Disposition", fmt.Sprintf(`attachment; filename="%s"`, name))
 	http.ServeFile(w, r, path)
 }
+
+// removeUserBackups deletes the server backups of a deleted account (FLAW-37). The system snapshots
+// still contain it until they expire (BACKUP retention).
+func (s *Server) removeUserBackups(userID string) {
+	dir := storage.UserBackupDir(s.backupRoot(), userID)
+	// Only ever one user's own folder, never backups/users itself (IDs like "." or "/")
+	if base := filepath.Base(dir); userID == "" || base == "." || base == "users" || filepath.Dir(dir) != filepath.Join(s.backupRoot(), "users") {
+		return
+	}
+	if err := os.RemoveAll(dir); err != nil {
+		slog.Warn("Backups des gelöschten Kontos konnten nicht entfernt werden", "user_id", userID, "error", err)
+	}
+}