ICYLAB Billing Notification + Manual Sync Hotfix v6.17.1

Files:
- services/notifications/NotificationService.php
- services/facto/FactoDocumentsSyncService.php
- api/dashboard/force_sync_facto.php
- assets/js/app.js
- scripts/billing_notification_sync_diagnostic_v6_17_1.php

Fixes:
- Removes PHP 7.4 typed properties from NotificationService for PHP 7.1+ compatibility.
- Restores FACTO direction mapping: 1 = issued, 0 = received.
- Preserves exclusion of production test document 862.
- Makes manual sync endpoint return valid JSON for Throwable errors.
- Prevents notification failures from masking the primary sync error.
- Shows the real backend error in the browser instead of a generic message.
