fix(query-engine): decouple backend health endpoint

[milindsrivastava1997]

Summary

• Add a backend-only configurable startup health endpoint.
• Use Prometheus /-/ready and VictoriaMetrics /health in experiment and Grafana demo wiring.
• Preserve ASAPQuery /api/v1/status/runtimeinfo and earliest timestamp metadata.
• Allocate dynamic ports for Prometheus forwarding test mocks so query-engine test targets can run in parallel.

Validation

• Cargo fmt, cargo check, and cargo clippy pass.
• cargo test --workspace passes.
• Python syntax and lint hooks pass.

The stale fake-exporter image issue observed in the VictoriaMetrics experiment is separate from this PR.