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/tenancy/tests/unit/test_store_schema.py

@@ -271,11 +271,13 @@ class TestStoreDetailResponseSchema:
             "owner_username": "owner",
             "contact_email": "contact@techstore.com",
             "contact_email_inherited": False,
+            "owner_user_id": 42,
         }
         response = StoreDetailResponse(**data)
         assert response.merchant_name == "Tech Corp"
         assert response.owner_email == "owner@techcorp.com"
         assert response.contact_email_inherited is False
+        assert response.owner_user_id == 42
 
 
 @pytest.mark.unit