Add Docker Compose deployment

- Dockerfile: Node.js 20 Alpine, single-stage build
- docker-compose.yml: backend + nginx, named volume for SQLite
- server.js: updated for Docker (env vars for PORT/DATA_DIR)
- nginx.conf: updated for Docker networking (backend hostname)
- README.md: full deployment instructions

deploy/.dockerignore

@@ -0,0 +1,4 @@
+node_modules
+npm-debug.log
+.git
+.gitignore