test(e2e): ProxyHostResponse.domain should be domain_names #1908

What: ProxyHostResponse in tests/utils/api-helpers.ts declares a domain field, but the backend returns domain_names (backend/internal/models/proxy_host.go). Code reading .domain gets undefined.

Where: tests/utils/api-helpers.ts

Evidence: Found by a Playwright agent while fixing ACL leaks in #1906. The new deleteProxyHostByDomain helper in tests/utils/proxy-refusal-helpers.ts matches on domain_names directly.

Severity: medium

Suggested approach: Fix the type, then grep for .domain uses of this type and correct them.

Found in: #1906