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>

alembic.ini

@@ -3,7 +3,7 @@
 script_location = alembic
 prepend_sys_path = .
 version_path_separator = space
-version_locations = alembic/versions app/modules/billing/migrations/versions app/modules/cart/migrations/versions app/modules/catalog/migrations/versions app/modules/cms/migrations/versions app/modules/customers/migrations/versions app/modules/dev_tools/migrations/versions app/modules/inventory/migrations/versions app/modules/loyalty/migrations/versions app/modules/marketplace/migrations/versions app/modules/messaging/migrations/versions app/modules/orders/migrations/versions
+version_locations = alembic/versions app/modules/billing/migrations/versions app/modules/cart/migrations/versions app/modules/catalog/migrations/versions app/modules/cms/migrations/versions app/modules/customers/migrations/versions app/modules/dev_tools/migrations/versions app/modules/inventory/migrations/versions app/modules/loyalty/migrations/versions app/modules/marketplace/migrations/versions app/modules/messaging/migrations/versions app/modules/orders/migrations/versions app/modules/tenancy/migrations/versions
 # This will be overridden by alembic\env.py using settings.database_url
 sqlalchemy.url =
 # for PROD: sqlalchemy.url = postgresql://username:password@localhost:5432/ecommerce_db