Add a small standalone Axum service for the shared anonymous top-ten table.
SQLite keeps the deployment self-contained, while one transaction inserts a
validated name and score and removes entries below the canonical top ten.
Expose a health endpoint and same-origin API, with an nginx proxy block and
run instructions beside the service. Keep the service outside the game crate
so native gameplay persistence remains unchanged.
Test Plan:
- `cargo test --manifest-path highscore-server/Cargo.toml` -- passed (3 tests)
- `cargo clippy --manifest-path highscore-server/Cargo.toml --all-targets --all-features -- -D warnings` -- passed
- `cargo +nightly fmt --manifest-path highscore-server/Cargo.toml -- --check` -- passed
- Live executable health, POST, and GET smoke test -- passed
- `git diff --cached --check` -- passed
