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/monitoring/services/capacity_forecast_service.py

@@ -74,7 +74,9 @@ class CapacityForecastService:
         # Resource metrics via provider pattern (avoids cross-module imports)
         start_of_month = now.replace(day=1, hour=0, minute=0, second=0, microsecond=0)
         platform = db.query(Platform).first()
-        platform_id = platform.id if platform else 1
+        if not platform:
+            raise ValueError("No platform found in database")
+        platform_id = platform.id
 
         stats = stats_aggregator.get_admin_stats_flat(
             db, platform_id,