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>

tests/unit/core/test_frontend_detector.py

@@ -65,10 +65,11 @@ class TestFrontendDetectorStore:
         result = FrontendDetector.detect(host="omsflow.lu", path="/store/dashboard/analytics")
         assert result == FrontendType.STORE
 
-    def test_stores_plural_not_store_dashboard(self):
-        """Test that /stores/ path is NOT store dashboard (it's storefront)."""
+    def test_stores_plural_is_not_storefront(self):
+        """Test that /stores/ path is NOT storefront (old pattern removed)."""
         result = FrontendDetector.detect(host="localhost", path="/stores/orion/storefront")
-        assert result == FrontendType.STOREFRONT
+        # /stores/ is no longer a storefront signal; /storefront/ is the canonical prefix
+        assert result != FrontendType.STOREFRONT
 
 
 @pytest.mark.unit
@@ -85,9 +86,9 @@ class TestFrontendDetectorStorefront:
         result = FrontendDetector.detect(host="localhost", path="/api/v1/storefront/cart")
         assert result == FrontendType.STOREFRONT
 
-    def test_detect_storefront_from_stores_path(self):
-        """Test storefront detection from /stores/ path (path-based store access)."""
-        result = FrontendDetector.detect(host="localhost", path="/stores/orion/products")
+    def test_detect_storefront_from_storefront_path(self):
+        """Test storefront detection from /storefront/ path (path-based store access)."""
+        result = FrontendDetector.detect(host="localhost", path="/storefront/orion/products")
         assert result == FrontendType.STOREFRONT
 
     def test_detect_storefront_from_store_subdomain(self):