chore: production-ready Dockerfile and docker-compose for engine + dashboar

[cokehill]

Summary

stellar-notify must be self-hostable with a single docker-compose up -d. The Docker setup
should follow security best practices (non-root user, minimal image).

Acceptance criteria

• Dockerfile — multi-stage build (deps → runtime), non-root user stellar
• Persistent volume mounted at /app/data for SQLite
• HEALTHCHECK using wget against /api/health
• dashboard/Dockerfile — Next.js standalone output
• docker-compose.yml — engine + dashboard services, shared volume, env_file
• docker-compose up -d starts cleanly with only .env configured
• README "Quick start" section documents the two-command flow

Notes

Use node:20-alpine base image.
better-sqlite3 requires native build — ensure python3 and make are available in build stage.