repo: expose storage health and trim stale docs

[argahsuknesib]

Summary

• expose storage background flush failures via the HTTP health endpoint
• remove the in-repo janus-dashboard placeholder and keep the dashboard boundary external
• trim stale design and milestone documents while updating current docs to point at the remaining sources of truth

Testing

• cargo fmt --all
• cargo test --test http_server_integration_test --all-features
• cargo test --test segmented_storage_error_test --all-features
• cargo test --test janus_cli_test --all-features
• cargo test --all-features
• cargo clippy --all-targets --all-features -- -D warnings