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/billing_service.py

@@ -88,21 +88,22 @@ class BillingService:
 
         return tier_list, tier_order
 
-    def get_tier_by_code(self, db: Session, tier_code: str) -> SubscriptionTier:
+    def get_tier_by_code(
+        self, db: Session, tier_code: str, platform_id: int | None = None
+    ) -> SubscriptionTier:
         """
-        Get a tier by its code.
+        Get a tier by its code, optionally scoped to a platform.
 
         Raises:
             TierNotFoundException: If tier doesn't exist
         """
-        tier = (
-            db.query(SubscriptionTier)
-            .filter(
-                SubscriptionTier.code == tier_code,
-                SubscriptionTier.is_active == True,  # noqa: E712
-            )
-            .first()
+        query = db.query(SubscriptionTier).filter(
+            SubscriptionTier.code == tier_code,
+            SubscriptionTier.is_active == True,  # noqa: E712
         )
+        if platform_id is not None:
+            query = query.filter(SubscriptionTier.platform_id == platform_id)
+        tier = query.first()
 
         if not tier:
             raise TierNotFoundException(tier_code)
@@ -133,7 +134,7 @@ class BillingService:
         if not stripe_service.is_configured:
             raise PaymentSystemNotConfiguredException()
 
-        tier = self.get_tier_by_code(db, tier_code)
+        tier = self.get_tier_by_code(db, tier_code, platform_id=platform_id)
 
         price_id = (
             tier.stripe_price_annual_id
@@ -410,7 +411,7 @@ class BillingService:
         if not subscription or not subscription.stripe_subscription_id:
             raise NoActiveSubscriptionException()
 
-        tier = self.get_tier_by_code(db, new_tier_code)
+        tier = self.get_tier_by_code(db, new_tier_code, platform_id=platform_id)
 
         price_id = (
             tier.stripe_price_annual_id