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/routes/api/admin.py

@@ -25,6 +25,7 @@ from .admin_merchants import admin_merchants_router
 from .admin_platforms import admin_platforms_router
 from .admin_stores import admin_stores_router
 from .admin_store_domains import admin_store_domains_router
+from .admin_merchant_domains import admin_merchant_domains_router
 from .admin_modules import router as admin_modules_router
 from .admin_module_config import router as admin_module_config_router
 
@@ -38,5 +39,6 @@ admin_router.include_router(admin_merchants_router, tags=["admin-merchants"])
 admin_router.include_router(admin_platforms_router, tags=["admin-platforms"])
 admin_router.include_router(admin_stores_router, tags=["admin-stores"])
 admin_router.include_router(admin_store_domains_router, tags=["admin-store-domains"])
+admin_router.include_router(admin_merchant_domains_router, tags=["admin-merchant-domains"])
 admin_router.include_router(admin_modules_router, tags=["admin-modules"])
 admin_router.include_router(admin_module_config_router, tags=["admin-module-config"])