P2-04 — Multi-backend per vhost with health checks

[bihius]

Context

Today each vhost has a single backend_url. Real reverse proxies load balance across multiple backends with health checks.

To do

• Model a Backend entity related to VHost as one-to-many
• Expose health check config
• Generate HAProxy config accordingly

Definition of done

• A vhost can point at two backends with active health checks