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>

middleware/storefront_access.py

@@ -129,23 +129,22 @@ class StorefrontAccessMiddleware(BaseHTTPMiddleware):
         return await call_next(request)
 
     def _get_subscription(self, db, store, request):
-        """Resolve subscription, handling multi-platform stores correctly."""
+        """Resolve subscription for the detected platform. No fallback."""
         from app.modules.billing.services.subscription_service import (
             subscription_service,
         )
 
         platform = getattr(request.state, "platform", None)
 
-        # If we have a detected platform, check subscription for THAT platform
-        if platform:
-            sub = subscription_service.get_merchant_subscription(
-                db, store.merchant_id, platform.id
+        if not platform:
+            logger.warning(
+                f"[STOREFRONT_ACCESS] No platform context for store '{store.subdomain}'"
             )
-            if sub:
-                return sub
+            return None
 
-        # Fallback: use store's primary platform (via StorePlatform)
-        return subscription_service.get_subscription_for_store(db, store.id)
+        return subscription_service.get_merchant_subscription(
+            db, store.merchant_id, platform.id
+        )
 
     def _render_unavailable(
         self, request: Request, reason: str, store=None