feat(tenancy): add merchant-level domain with store override

Merchants can now register domains (e.g., myloyaltyprogram.lu) that all
their stores inherit. Individual stores can override with their own custom
domain. Resolution priority: StoreDomain > MerchantDomain > subdomain.

- Add MerchantDomain model, schema, service, and admin API endpoints
- Add merchant domain fallback in platform and store context middleware
- Add Merchant.primary_domain and Store.effective_domain properties
- Add Alembic migration for merchant_domains table
- Update loyalty user journey docs with subscription & domain setup flow
- Add unit tests (50 passing) and integration tests (15 passing)

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

tests/integration/middleware/conftest.py

@@ -65,12 +65,13 @@ def client(db):
     # Patch get_db in middleware modules - they have their own imports
     # The middleware calls: db_gen = get_db(); db = next(db_gen)
     # Also patch settings.platform_domain so subdomain detection works with test hosts
-    with patch("middleware.store_context.get_db", override_get_db):
-        with patch("middleware.theme_context.get_db", override_get_db):
-            with patch("middleware.store_context.settings") as mock_settings:
-                mock_settings.platform_domain = "platform.com"
-                client = TestClient(app)
-                yield client
+    with patch("middleware.platform_context.get_db", override_get_db):
+        with patch("middleware.store_context.get_db", override_get_db):
+            with patch("middleware.theme_context.get_db", override_get_db):
+                with patch("middleware.store_context.settings") as mock_settings:
+                    mock_settings.platform_domain = "platform.com"
+                    client = TestClient(app)
+                    yield client
 
     # Clean up
     if get_db in app.dependency_overrides: