perf: add defer to scripts and lazy loading to images

Add defer attribute to 145 <script> tags across 103 template files
(PERF-067) and loading="lazy" to 22 <img> tags across 13 template
files (PERF-058). Both improve page load performance.

Validator totals: 0 errors, 2 warnings, 1360 info (down from 1527).

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

app/modules/inventory/templates/inventory/admin/inventory.html

@@ -599,5 +599,5 @@
 {% endblock %}
 
 {% block extra_scripts %}
-<script src="{{ url_for('inventory_static', path='admin/js/inventory.js') }}"></script>
+<script defer src="{{ url_for('inventory_static', path='admin/js/inventory.js') }}"></script>
 {% endblock %}

app/modules/inventory/templates/inventory/store/inventory.html

@@ -370,5 +370,5 @@
 {% endblock %}
 
 {% block extra_scripts %}
-<script src="{{ url_for('inventory_static', path='store/js/inventory.js') }}"></script>
+<script defer src="{{ url_for('inventory_static', path='store/js/inventory.js') }}"></script>
 {% endblock %}