scan-health: support custom status codes as healthy

[mataeil]

Some APIs return non-200 status codes for healthy responses (e.g., 204 No Content, 302 redirect for auth-gated endpoints).

Currently scan-health treats only HTTP 200 as healthy. This should be configurable per endpoint.

Proposed config change:
health_endpoints could accept objects instead of strings:
{ "url": "https://api.example.com/health", "healthy_codes": [200, 204] }

String entries would continue to work (default to [200]).