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/conftest.py

@@ -176,4 +176,5 @@ pytest_plugins = [
     "tests.fixtures.message_fixtures",
     "tests.fixtures.testing_fixtures",
     "tests.fixtures.content_page_fixtures",
+    "tests.fixtures.merchant_domain_fixtures",
 ]