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/tenancy/routes/api/admin_stores.py

@@ -72,6 +72,7 @@ def create_store(
         merchant_contact_phone=store.merchant.contact_phone,
         merchant_website=store.merchant.website,
         # Owner info (from merchant)
+        owner_user_id=store.merchant.owner.id,
         owner_email=store.merchant.owner.email,
         owner_username=store.merchant.owner.username,
         login_url=f"http://localhost:8000/store/{store.subdomain}/login",
@@ -143,6 +144,7 @@ def _build_store_detail_response(store) -> StoreDetailResponse:
         # Merchant info
         merchant_name=store.merchant.name,
         # Owner details (from merchant)
+        owner_user_id=store.merchant.owner_user_id,
         owner_email=store.merchant.owner.email,
         owner_username=store.merchant.owner.username,
         # Resolved contact info with inheritance flags