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>

app/modules/tenancy/models/__init__.py

@@ -34,6 +34,7 @@ from app.modules.tenancy.models.platform import Platform
 from app.modules.tenancy.models.platform_module import PlatformModule
 from app.modules.tenancy.models.user import User, UserRole
 from app.modules.tenancy.models.store import Role, Store, StoreUser, StoreUserType
+from app.modules.tenancy.models.merchant_domain import MerchantDomain
 from app.modules.tenancy.models.store_domain import StoreDomain
 from app.modules.tenancy.models.store_platform import StorePlatform
 
@@ -59,6 +60,8 @@ __all__ = [
     "StoreUser",
     "StoreUserType",
     "Role",
+    # Merchant configuration
+    "MerchantDomain",
     # Store configuration
     "StoreDomain",
     "StorePlatform",