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/exceptions/handler.py

@@ -388,7 +388,7 @@ def _redirect_to_login(request: Request) -> RedirectResponse:
     Uses FrontendType detection to determine admin vs store vs storefront login.
     Properly handles multi-access routing (domain, subdomain, path-based).
     """
-    frontend_type = get_frontend_type(request)
+    frontend_type = get_frontend_type(request) or FrontendType.PLATFORM
 
     if frontend_type == FrontendType.ADMIN:
         logger.debug("Redirecting to /admin/login")