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/templates/merchant/base.html

@@ -60,7 +60,7 @@
     <script defer src="{{ url_for('static', path='shared/js/icons.js') }}"></script>
 
     <!-- 3. THIRD: Alpine.js Base Data -->
-    <script src="{{ url_for('core_static', path='merchant/js/init-alpine.js') }}"></script>
+    <script src="{{ static_v(request, 'core_static', path='merchant/js/init-alpine.js') }}"></script>
 
     <!-- 4. FOURTH: Utils (standalone utilities) -->
     <script defer src="{{ url_for('static', path='shared/js/utils.js') }}"></script>