feat(static-assets): cache-bust JS/CSS via ?v=<commit-sha>, immutable in prod

Adds a `static_v(request, name, path=...)` Jinja helper that appends
?v=<commit-sha> from app.core.build_info, plus a CachedStaticFiles
subclass that serves Cache-Control: public, max-age=31536000, immutable
in production and no-cache in development. Browsers refetch JS/CSS
automatically on every deploy without the user having to hard-reload.

- New: app/core/static_files.py (CachedStaticFiles)
- Updated: app/templates_config.py (static_v helper)
- Updated: main.py (use CachedStaticFiles for *_static mounts)
- Codemod: 143 url_for('*_static', path='*.js'|'*.css') → static_v(...)
  across 123 templates. Images/fonts/JSON locales intentionally
  unchanged (out of scope).
- Arch rule: FE-024 (warning) flags raw url_for on JS/CSS to prevent
  drift. Note: FE-008 was already taken by the number_stepper rule.
- docs/proposals/static-asset-cache-busting.md marked Done.

Closes plan from docs/proposals/static-asset-cache-busting.md.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

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

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