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/modules/billing/services/__init__.py

@@ -21,6 +21,10 @@ from app.modules.billing.services.platform_pricing_service import (
     PlatformPricingService,
     platform_pricing_service,
 )
+from app.modules.billing.services.store_platform_sync_service import (
+    StorePlatformSync,
+    store_platform_sync,
+)
 from app.modules.billing.services.stripe_service import (
     StripeService,
     stripe_service,
@@ -42,6 +46,8 @@ from app.modules.billing.services.usage_service import (
 __all__ = [
     "SubscriptionService",
     "subscription_service",
+    "StorePlatformSync",
+    "store_platform_sync",
     "StripeService",
     "stripe_service",
     "AdminSubscriptionService",