feat: platform-aware storefront routing and billing improvements

Overhaul storefront URL routing to be platform-aware:
- Dev: /platforms/{code}/storefront/{store_code}/
- Prod: subdomain.platform.lu/ (internally rewritten to /storefront/)
- Add subdomain detection in PlatformContextMiddleware
- Add /storefront/ path rewrite for prod mode (subdomain/custom domain)
- Remove all silent platform fallbacks (platform_id=1)
- Add require_platform dependency for clean endpoint validation
- Update route registration, templates, module definitions, base_url calc
- Update StoreContextMiddleware for /storefront/ path detection
- Remove /stores/ from FrontendDetector STOREFRONT_PATH_PREFIXES

Billing service improvements:
- Add store_platform_sync_service to keep store_platforms in sync
- Make tier lookups platform-aware across billing services
- Add tiers for all platforms in seed data
- Add demo subscriptions to seed

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

app/templates/storefront/errors/401.html

@@ -7,16 +7,16 @@
 {% block title %}401 - Authentication Required{% endblock %}
 
 {% block action_buttons %}
-<a href="{{ base_url }}storefront/account/login"
+<a href="{{ base_url }}account/login"
    class="inline-flex items-center px-8 py-4 rounded-xl font-semibold text-white bg-theme-primary hover:opacity-90 hover:-translate-y-0.5 transition-all shadow-lg">
     Log In
 </a>
-<a href="{{ base_url }}storefront/account/register"
+<a href="{{ base_url }}account/register"
    class="inline-flex items-center px-8 py-4 rounded-xl font-semibold text-theme-primary border-2 border-theme-primary hover:bg-theme-primary hover:text-white hover:-translate-y-0.5 transition-all">
     Create Account
 </a>
 {% endblock %}
 
 {% block support_link %}
-Don't have an account? <a href="{{ base_url }}storefront/account/register" class="text-theme-primary font-semibold hover:underline">Sign up now</a>
+Don't have an account? <a href="{{ base_url }}account/register" class="text-theme-primary font-semibold hover:underline">Sign up now</a>
 {% endblock %}