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/core/frontend_detector.py

@@ -46,7 +46,6 @@ class FrontendDetector:
     STOREFRONT_PATH_PREFIXES = (
         "/storefront",
         "/api/v1/storefront",
-        "/stores/",  # Path-based store access
     )
     MERCHANT_PATH_PREFIXES = ("/merchants", "/api/v1/merchants")
     PLATFORM_PATH_PREFIXES = ("/api/v1/platform",)