Rust (axum + tokio). Implements ../PROTOCOL.md: WebSocket rooms that live entirely in memory and a static file server for the built frontend — the whole app is one binary plus a dist/ folder.

Run it

# build the frontend first so there is something to serve
cd ../frontend && npm run build && cd ../backend

cargo run            # http://localhost:8080

PORT — listen port (default 8080)
STATIC_DIR — built frontend location (default ../frontend/dist)

During frontend development you can instead run npm run dev with VITE_WS=1 and Vite proxies /ws here.

How it stays small

No database, by design. A Room is a deck, a revealed flag, a round counter and a Vec of players — a few hundred bytes. All rooms sit in one Mutex<HashMap>. When the last socket of a room closes, the room is removed; restart the server and everything is gone, which is the point.
One task per socket, nothing else. Each connection is a single tokio::select! loop (snapshots out, messages in, a 30s keepalive ping). No per-room goroutine-style actors, no background jobs, no timers per room.
Snapshots are serialized once per change (borrowed data, no clones) and fanned out through a per-room broadcast channel with a small buffer.

Files

src/main.rs   routes + static serving with SPA fallback (~40 lines)
src/room.rs   room state and the game rules (vote/reveal/reset/deck)
src/ws.rs     socket lifecycle: hello → select loop → disconnect cleanup

Rules enforced here (mirroring PROTOCOL.md): votes are hidden until reveal and rejected after it; reveal needs at least one vote; reset/deck-change start a fresh round; a refresh re-claims the same seat without losing the vote; watchers (watch handshake — the TV view) receive everything and may send nothing.