Implement backend-node minimal WebSocket relay package

[OpenCodeEngineer]

Summary\nImplement a new package at for a minimal Node.js + TypeScript backend relay using Express, ws, and better-sqlite3.\n\n## Requirements\n- Exact package.json scripts/deps/devDeps as specified\n- tsconfig: ES2020, CommonJS, outDir dist, strict\n- Implement REST routes, SQLite schema + idempotent seed\n- Implement WebSocket upgrades for /ws/setup and /ws/widget/:embedToken with auth handshake gating\n- Add safe typings and error handling\n- Run local verification: npm install and npm run build in package dir\n\n## Acceptance\n- Files created under \n- Build succeeds\n- Output captured