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/handlers_auth.go

@@ -385,12 +385,14 @@ func (s *Server) handleDeleteOwnAccount(w http.ResponseWriter, r *http.Request)
 		writeError(w, status, err.Error())
 		return
 	}
+	s.removeUserBackups(user.ID)
 	slog.Info("Benutzer hat sein Konto gelöscht", "user_id", user.ID)
 	writeJSON(w, http.StatusOK, map[string]bool{"deleted": true})
 }
 
 // importUpload reads the multipart field "file" (max. storage.MaxImportUploadBytes).
 func importUpload(w http.ResponseWriter, r *http.Request) ([]byte, string, error) {
+	allowLongTransfer(w)
 	r.Body = http.MaxBytesReader(w, r.Body, storage.MaxImportUploadBytes+1<<20)
 	file, header, err := r.FormFile("file")
 	if err != nil {
@@ -456,6 +458,10 @@ func (s *Server) handleUpdateUserSettings(w http.ResponseWriter, r *http.Request
 	}
 	changesPassword := req.Password != nil && strings.TrimSpace(*req.Password) != ""
 	changesEmail := req.Email != nil && !strings.EqualFold(strings.TrimSpace(*req.Email), before.Email)
+	if (changesPassword || changesEmail) && s.cfg.DemoMode {
+		writeError(w, http.StatusForbidden, "In der Demo deaktiviert")
+		return
+	}
 	if (changesPassword || changesEmail) && !s.checkCurrentPassword(w, r, before, req.CurrentPassword) {
 		return
 	}