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/routes/pages/store.py

@@ -184,7 +184,9 @@ async def store_content_page(
     store = getattr(request.state, "store", None)
     platform = getattr(request.state, "platform", None)
     store_id = store.id if store else None
-    platform_id = platform.id if platform else 1
+    if not platform:
+        raise HTTPException(status_code=400, detail="Platform context required")
+    platform_id = platform.id
 
     # Load content page from database (store override → platform default)
     page = content_page_service.get_page_for_store(