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/routes/api/admin.py

@@ -16,7 +16,6 @@ from sqlalchemy.orm import Session
 
 from app.api.deps import get_current_admin_api, require_module_access
 from app.core.database import get_db
-from app.exceptions import ResourceNotFoundException
 from app.modules.billing.schemas import (
     BillingHistoryListResponse,
     BillingHistoryWithMerchant,
@@ -284,16 +283,7 @@ def get_subscription_for_store(
     store -> merchant -> all platform subscriptions and returns a list
     of subscription entries with feature usage metrics.
     """
-    from app.modules.billing.services.feature_service import feature_service
-
-    # Resolve store to merchant
-    merchant_id, platform_ids = feature_service._get_merchant_and_platforms_for_store(db, store_id)
-    if merchant_id is None or not platform_ids:
-        raise ResourceNotFoundException("Store", str(store_id))
-
-    results = admin_subscription_service.get_merchant_subscriptions_with_usage(
-        db, merchant_id
-    )
+    results = admin_subscription_service.get_subscriptions_for_store(db, store_id)
     return {"subscriptions": results}