feat(highscores): add SQLite Axum service

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

tdkpin-rs/highscore-server/nginx.conf.example

@@ -0,0 +1,15 @@
+# Add this block inside the nginx server block that serves the game.
+location /api/highscores {
+    proxy_pass http://127.0.0.1:3000;
+    proxy_http_version 1.1;
+    proxy_set_header Host $host;
+    proxy_set_header X-Real-IP $remote_addr;
+    proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
+}
+
+# Optional health check for local monitoring.
+location = /healthz {
+    proxy_pass http://127.0.0.1:3000;
+    proxy_http_version 1.1;
+    proxy_set_header Host $host;
+}