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/cms/definition.py

@@ -39,7 +39,9 @@ def _get_platform_context(request: Any, db: Any, platform: Any) -> dict[str, Any
     """
     from app.modules.cms.services import content_page_service
 
-    platform_id = platform.id if platform else 1
+    if not platform:
+        return {"header_pages": [], "footer_pages": [], "legal_pages": []}
+    platform_id = platform.id
 
     header_pages = []
     footer_pages = []
@@ -73,7 +75,9 @@ def _get_storefront_context(request: Any, db: Any, platform: Any) -> dict[str, A
     from app.modules.cms.services import content_page_service
 
     store = getattr(request.state, "store", None)
-    platform_id = platform.id if platform else 1
+    if not platform:
+        return {"header_pages": [], "footer_pages": [], "legal_pages": []}
+    platform_id = platform.id
 
     header_pages = []
     footer_pages = []