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/integration/test_admin_routes.py

@@ -507,3 +507,56 @@ class TestAdminBillingHistory:
         )
         assert response.status_code == 200
         assert response.json()["total"] == 0
+
+
+# ============================================================================
+# Store Subscription Convenience Endpoint
+# ============================================================================
+
+
+class TestAdminStoreSubscription:
+    """Tests for GET /api/v1/admin/subscriptions/store/{store_id}."""
+
+    def test_get_subscriptions_for_store(
+        self, client, super_admin_headers, rt_subscription, rt_store
+    ):
+        """Returns subscriptions when store has a merchant with subscriptions."""
+        response = client.get(
+            f"{BASE}/store/{rt_store.id}",
+            headers=super_admin_headers,
+        )
+        assert response.status_code == 200
+        data = response.json()
+        assert "subscriptions" in data
+        assert len(data["subscriptions"]) >= 1
+
+    def test_get_subscriptions_for_nonexistent_store(
+        self, client, super_admin_headers
+    ):
+        """Returns 404 for non-existent store ID."""
+        response = client.get(
+            f"{BASE}/store/999999",
+            headers=super_admin_headers,
+        )
+        assert response.status_code == 404
+
+
+@pytest.fixture
+def rt_store(db, rt_merchant):
+    """Create a store for route tests."""
+    from app.modules.tenancy.models import Store
+
+    store = Store(
+        merchant_id=rt_merchant.id,
+        store_code=f"RT_{uuid.uuid4().hex[:6].upper()}",
+        name="Route Test Store",
+        subdomain=f"rt-{uuid.uuid4().hex[:8]}",
+        is_active=True,
+        is_verified=True,
+        created_at=datetime.now(UTC),
+        updated_at=datetime.now(UTC),
+    )
+    db.add(store)
+    db.commit()
+    db.refresh(store)
+    return store