feat(middleware): harden routing with fail-closed policy, custom subdomain management, and perf fixes

- Fix IPv6 host parsing with _strip_port() utility
- Remove dangerous StorePlatform→Store.subdomain silent fallback
- Close storefront gate bypass when frontend_type is None
- Add custom subdomain management UI and API for stores
- Add domain health diagnostic tool
- Convert db.add() in loops to db.add_all() (24 PERF-006 fixes)
- Add tests for all new functionality (18 subdomain service tests)
- Add .github templates for validator compliance

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

app/modules/tenancy/routes/api/admin.py

@@ -27,6 +27,7 @@ from .admin_platform_users import admin_platform_users_router
 from .admin_platforms import admin_platforms_router
 from .admin_store_domains import admin_store_domains_router
 from .admin_store_roles import admin_store_roles_router
+from .admin_store_subdomains import admin_store_subdomains_router
 from .admin_stores import admin_stores_router
 from .admin_users import admin_users_router
 from .user_account import admin_account_router
@@ -42,6 +43,7 @@ router.include_router(admin_merchants_router, tags=["admin-merchants"])
 router.include_router(admin_platforms_router, tags=["admin-platforms"])
 router.include_router(admin_stores_router, tags=["admin-stores"])
 router.include_router(admin_store_domains_router, tags=["admin-store-domains"])
+router.include_router(admin_store_subdomains_router, tags=["admin-store-subdomains"])
 router.include_router(admin_store_roles_router, tags=["admin-store-roles"])
 router.include_router(admin_merchant_domains_router, tags=["admin-merchant-domains"])
 router.include_router(admin_modules_router, tags=["admin-modules"])