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/billing/templates/billing/admin/billing-history.html

@@ -203,5 +203,5 @@
 {% endblock %}
 
 {% block extra_scripts %}
-<script src="{{ url_for('billing_static', path='admin/js/billing-history.js') }}"></script>
+<script defer src="{{ url_for('billing_static', path='admin/js/billing-history.js') }}"></script>
 {% endblock %}

app/modules/billing/templates/billing/admin/subscription-tiers.html

@@ -474,5 +474,5 @@
 {% endblock %}
 
 {% block extra_scripts %}
-<script src="{{ url_for('billing_static', path='admin/js/subscription-tiers.js') }}"></script>
+<script defer src="{{ url_for('billing_static', path='admin/js/subscription-tiers.js') }}"></script>
 {% endblock %}

app/modules/billing/templates/billing/admin/subscriptions.html

@@ -316,5 +316,5 @@
 {% endblock %}
 
 {% block extra_scripts %}
-<script src="{{ url_for('billing_static', path='admin/js/subscriptions.js') }}"></script>
+<script defer src="{{ url_for('billing_static', path='admin/js/subscriptions.js') }}"></script>
 {% endblock %}

app/modules/billing/templates/billing/platform/signup.html

@@ -6,7 +6,7 @@
 
 {% block extra_head %}
 {# Stripe.js for payment #}
-<script src="https://js.stripe.com/v3/"></script>
+<script defer src="https://js.stripe.com/v3/"></script>
 {% endblock %}
 
 {% block content %}

app/modules/billing/templates/billing/store/billing.html

@@ -402,5 +402,5 @@
 {% endblock %}
 
 {% block extra_scripts %}
-<script src="/static/modules/billing/store/js/billing.js"></script>
+<script defer src="/static/modules/billing/store/js/billing.js"></script>
 {% endblock %}