Fix QZ Tray library loading and add pairing keys API endpoint

- Fix QZ Tray library loading errors (net::ERR_NAME_NOT_RESOLVED) by switching from CDN to local static file
- Add /labels/api/pairing-keys endpoint to fetch valid printer pairing keys
- Update print_labels.html to use local js/qz-tray.js
- Update print_lost_labels.html to use local js/qz-tray.js
- Update print_lost_labels_new.html to use correct path js/qz-tray.js
- Update fg_scan.html to use local js/qz-tray.js
- API returns active pairing keys from qz_pairing_keys table for printer selection

app/templates/modules/quality/fg_scan.html

@@ -4,7 +4,7 @@
 
 {% block extra_css %}
 <link rel="stylesheet" href="{{ url_for('static', filename='css/fg_scan.css') }}">
-<script src="https://cdn.jsdelivr.net/npm/qz-tray@2.1.0/qz-tray.js"></script>
+<script src="{{ url_for('static', filename='js/qz-tray.js') }}"></script>
 {% endblock %}
 
 {% block content %}