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>

tests/unit/middleware/test_storefront_access.py

@@ -430,51 +430,40 @@ class TestGetSubscription:
             )
             assert result is expected_sub
 
-    def test_falls_back_to_store_primary_platform(self):
-        """Test fallback to get_subscription_for_store when platform sub is None."""
+    def test_no_fallback_when_merchant_subscription_none(self):
+        """Test no fallback when get_merchant_subscription returns None."""
         middleware = StorefrontAccessMiddleware(app=None)
         store = _make_store(store_id=5, merchant_id=10)
         platform = _make_platform(platform_id=2)
         request = _make_request(store=store, platform=platform)
         mock_db = MagicMock()
-        fallback_sub = _make_subscription(tier_code="starter")
 
         with patch(
             "app.modules.billing.services.subscription_service.subscription_service"
         ) as mock_svc:
             mock_svc.get_merchant_subscription.return_value = None
-            mock_svc.get_subscription_for_store.return_value = fallback_sub
 
             result = middleware._get_subscription(mock_db, store, request)
 
             mock_svc.get_merchant_subscription.assert_called_once_with(
                 mock_db, 10, 2
             )
-            mock_svc.get_subscription_for_store.assert_called_once_with(
-                mock_db, 5
-            )
-            assert result is fallback_sub
+            assert result is None
 
-    def test_no_platform_uses_store_fallback(self):
-        """Test when no platform is detected, falls back to store-based lookup."""
+    def test_no_platform_returns_none(self):
+        """Test when no platform is detected, returns None (no fallback)."""
         middleware = StorefrontAccessMiddleware(app=None)
         store = _make_store(store_id=7)
         request = _make_request(store=store, platform=None)
         mock_db = MagicMock()
-        fallback_sub = _make_subscription()
 
         with patch(
             "app.modules.billing.services.subscription_service.subscription_service"
         ) as mock_svc:
-            mock_svc.get_subscription_for_store.return_value = fallback_sub
-
             result = middleware._get_subscription(mock_db, store, request)
 
             mock_svc.get_merchant_subscription.assert_not_called()
-            mock_svc.get_subscription_for_store.assert_called_once_with(
-                mock_db, 7
-            )
-            assert result is fallback_sub
+            assert result is None
 
 
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