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/subscription_service.py

@@ -82,17 +82,20 @@ class SubscriptionService:
     # Tier Information
     # =========================================================================
 
-    def get_tier_by_code(self, db: Session, tier_code: str) -> SubscriptionTier | None:
-        """Get subscription tier by code."""
-        return (
-            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 | None:
+        """Get 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)
+        return query.first()
 
-    def get_tier_id(self, db: Session, tier_code: str) -> int | None:
+    def get_tier_id(
+        self, db: Session, tier_code: str, platform_id: int | None = None
+    ) -> int | None:
         """Get tier ID from tier code. Returns None if tier not found."""
-        tier = self.get_tier_by_code(db, tier_code)
+        tier = self.get_tier_by_code(db, tier_code, platform_id=platform_id)
         return tier.id if tier else None
 
     def get_all_tiers(
@@ -254,7 +257,7 @@ class SubscriptionService:
             trial_ends_at = None
             status = SubscriptionStatus.ACTIVE.value
 
-        tier_id = self.get_tier_id(db, tier_code)
+        tier_id = self.get_tier_id(db, tier_code, platform_id=platform_id)
 
         subscription = MerchantSubscription(
             merchant_id=merchant_id,
@@ -271,6 +274,15 @@ class SubscriptionService:
         db.flush()
         db.refresh(subscription)
 
+        # Sync store_platforms for all merchant stores
+        from app.modules.billing.services.store_platform_sync_service import (
+            store_platform_sync,
+        )
+
+        store_platform_sync.sync_store_platforms_for_merchant(
+            db, merchant_id, platform_id, is_active=True, tier_id=subscription.tier_id
+        )
+
         logger.info(
             f"Created subscription for merchant {merchant_id} on platform {platform_id} "
             f"(tier={tier_code}, status={status})"
@@ -305,7 +317,7 @@ class SubscriptionService:
         subscription = self.get_subscription_or_raise(db, merchant_id, platform_id)
 
         old_tier_id = subscription.tier_id
-        new_tier = self.get_tier_by_code(db, new_tier_code)
+        new_tier = self.get_tier_by_code(db, new_tier_code, platform_id=platform_id)
         if not new_tier:
             raise ValueError(f"Tier '{new_tier_code}' not found")
 
@@ -366,6 +378,15 @@ class SubscriptionService:
         db.flush()
         db.refresh(subscription)
 
+        # Sync store_platforms for all merchant stores
+        from app.modules.billing.services.store_platform_sync_service import (
+            store_platform_sync,
+        )
+
+        store_platform_sync.sync_store_platforms_for_merchant(
+            db, merchant_id, platform_id, is_active=True
+        )
+
         logger.info(
             f"Reactivated subscription for merchant {merchant_id} "
             f"on platform {platform_id}"