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/base.html

@@ -76,11 +76,11 @@
         {# Action Buttons #}
         <div class="flex gap-4 justify-center flex-wrap mt-8">
             {% 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">
                 Continue Shopping
             </a>
-            <a href="{{ base_url }}storefront/contact"
+            <a href="{{ base_url }}contact"
                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">
                 Contact Us
             </a>
@@ -92,7 +92,7 @@
         {# Support Link #}
         <div class="mt-10 pt-8 border-t border-gray-200 text-sm text-gray-500">
             {% block support_link %}
-            Need help? <a href="{{ base_url }}storefront/contact" class="text-theme-primary font-semibold hover:underline">Contact our support team</a>
+            Need help? <a href="{{ base_url }}contact" class="text-theme-primary font-semibold hover:underline">Contact our support team</a>
             {% endblock %}
         </div>