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/justfile

@@ -19,6 +19,15 @@ web-build:
 web-serve: web-build
     python3 -m http.server 8000 --directory web
 
+highscore-server:
+    cargo run --manifest-path highscore-server/Cargo.toml
+
+test-highscore-server:
+    cargo test --manifest-path highscore-server/Cargo.toml
+
+clippy-highscore-server:
+    cargo clippy --manifest-path highscore-server/Cargo.toml --all-targets --all-features -- -D warnings
+
 fmt:
     cargo +nightly fmt
     tombi format
@@ -32,10 +41,10 @@ _fix:
 
 fix: _fix fmt
 
-clippy:
+clippy: clippy-highscore-server
     cargo clippy --workspace --all-targets --all-features -- -D warnings
 
-test:
+test: test-highscore-server
     cargo test --workspace --all-targets --all-features
 
 clean: