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/tests/unit/test_subscription_service.py

@@ -502,7 +502,7 @@ class TestSubscriptionServiceReactivate:
 
 
 @pytest.fixture
-def billing_tier_essential(db):
+def billing_tier_essential(db, test_platform):
     """Create essential subscription tier."""
     tier = SubscriptionTier(
         code="essential",
@@ -513,6 +513,7 @@ def billing_tier_essential(db):
         display_order=1,
         is_active=True,
         is_public=True,
+        platform_id=test_platform.id,
     )
     db.add(tier)
     db.commit()
@@ -521,7 +522,7 @@ def billing_tier_essential(db):
 
 
 @pytest.fixture
-def billing_tiers(db):
+def billing_tiers(db, test_platform):
     """Create essential, professional, and business tiers."""
     tiers = [
         SubscriptionTier(
@@ -532,6 +533,7 @@ def billing_tiers(db):
             display_order=1,
             is_active=True,
             is_public=True,
+            platform_id=test_platform.id,
         ),
         SubscriptionTier(
             code="professional",
@@ -541,6 +543,7 @@ def billing_tiers(db):
             display_order=2,
             is_active=True,
             is_public=True,
+            platform_id=test_platform.id,
         ),
         SubscriptionTier(
             code="business",
@@ -550,6 +553,7 @@ def billing_tiers(db):
             display_order=3,
             is_active=True,
             is_public=True,
+            platform_id=test_platform.id,
         ),
     ]
     db.add_all(tiers)