Trigger immediate backend health check after add or enable

[ktrubilo9]

What

Trigger an immediate health check after a backend is added or enabled.

Previously, a new or enabled backend stayed inactive until the next periodic health-check cycle.

Changes

• trigger an async health check after adding an enabled backend
• trigger an async health check after enabling a backend
• keep failed backends inactive
• keep the existing periodic health checks
• add tests for add and enable flows

Tests

• add enabled backend + healthy → active
• add enabled backend + unhealthy → inactive
• enable disabled backend + healthy → active
• enable disabled backend + unhealthy → inactive

go test ./...

Closes #21