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/handlers/stripe_webhook.py

@@ -380,6 +380,24 @@ class StripeWebhookHandler:
                     f"Tier changed to {tier.code} for merchant {subscription.merchant_id}"
                 )
 
+        # Sync store_platforms based on subscription status
+        from app.modules.billing.services.store_platform_sync_service import (
+            store_platform_sync,
+        )
+
+        active_statuses = {
+            SubscriptionStatus.TRIAL.value,
+            SubscriptionStatus.ACTIVE.value,
+            SubscriptionStatus.PAST_DUE.value,
+            SubscriptionStatus.CANCELLED.value,
+        }
+        store_platform_sync.sync_store_platforms_for_merchant(
+            db,
+            subscription.merchant_id,
+            subscription.platform_id,
+            is_active=subscription.status in active_statuses,
+        )
+
         logger.info(f"Subscription updated for merchant {subscription.merchant_id}")
         return {"action": "updated", "merchant_id": subscription.merchant_id}
 
@@ -435,6 +453,15 @@ class StripeWebhookHandler:
         if addon_count > 0:
             logger.info(f"Cancelled {addon_count} add-ons for merchant {merchant_id}")
 
+        # Deactivate store_platforms for the deleted subscription's platform
+        from app.modules.billing.services.store_platform_sync_service import (
+            store_platform_sync,
+        )
+
+        store_platform_sync.sync_store_platforms_for_merchant(
+            db, merchant_id, subscription.platform_id, is_active=False
+        )
+
         logger.info(f"Subscription deleted for merchant {merchant_id}")
         return {
             "action": "cancelled",