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/billing/services/admin_subscription_service.py

@@ -56,13 +56,14 @@ class AdminSubscriptionService:
 
         return query.order_by(SubscriptionTier.display_order).all()
 
-    def get_tier_by_code(self, db: Session, tier_code: str) -> SubscriptionTier:
-        """Get a subscription tier by code."""
-        tier = (
-            db.query(SubscriptionTier)
-            .filter(SubscriptionTier.code == tier_code)
-            .first()
-        )
+    def get_tier_by_code(
+        self, db: Session, tier_code: str, platform_id: int | None = None
+    ) -> SubscriptionTier:
+        """Get a subscription tier by code, optionally scoped to a platform."""
+        query = db.query(SubscriptionTier).filter(SubscriptionTier.code == tier_code)
+        if platform_id is not None:
+            query = query.filter(SubscriptionTier.platform_id == platform_id)
+        tier = query.first()
 
         if not tier:
             raise TierNotFoundException(tier_code)
@@ -214,7 +215,7 @@ class AdminSubscriptionService:
                     db, merchant_id, platform_id, tier_code, sub.is_annual
                 )
             else:
-                tier = self.get_tier_by_code(db, tier_code)
+                tier = self.get_tier_by_code(db, tier_code, platform_id=platform_id)
                 sub.tier_id = tier.id
 
         for field, value in update_data.items():
@@ -350,6 +351,22 @@ class AdminSubscriptionService:
 
         return results
 
+    def get_subscriptions_for_store(
+        self, db: Session, store_id: int
+    ) -> list[dict]:
+        """Get subscriptions + feature usage for a store (resolves to merchant).
+
+        Convenience method for admin store detail page. Resolves
+        store -> merchant -> all platform subscriptions.
+        """
+        from app.modules.tenancy.models import Store
+
+        store = db.query(Store).filter(Store.id == store_id).first()
+        if not store or not store.merchant_id:
+            raise ResourceNotFoundException("Store", str(store_id))
+
+        return self.get_merchant_subscriptions_with_usage(db, store.merchant_id)
+
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