Adds a one-shot `--role seed` to the server binary (migrate → seed demo store → exit) and extracts the seeder into a shared `server::seed` module the bin + role + tests all call. Dockerfile splits ENTRYPOINT ["server"] + CMD ["--role","all"], so seeding is just `docker run <image> --role seed` — retiring the `--entrypoint /usr/local/bin/seed-demo` gotcha. Drops the seed-demo binary from the image (kept as a cargo-run convenience bin). Deploy docs + prod compose updated; the integration test now exercises the real seeder.