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/500.html

@@ -7,7 +7,7 @@
 {% block title %}500 - Something Went Wrong{% endblock %}
 
 {% block action_buttons %}
-<a href="{{ base_url }}storefront/"
+<a href="{{ base_url }}"
    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">
     Go to Home
 </a>
@@ -18,5 +18,5 @@
 {% endblock %}
 
 {% block support_link %}
-Issue persisting? <a href="{{ base_url }}storefront/contact" class="text-theme-primary font-semibold hover:underline">Let us know</a> and we'll help you out.
+Issue persisting? <a href="{{ base_url }}contact" class="text-theme-primary font-semibold hover:underline">Let us know</a> and we'll help you out.
 {% endblock %}