feat(backend): add Docker support for self-hosting

[joaoclaudio82]

Add a multi-stage Dockerfile that runs the backend as an HTTP server (apps/backend/express.js), so cards can be served outside Vercel. The image builds the core package, produces a production-only bundle via 'pnpm deploy', and runs as a non-root user with a healthcheck against /api/status/up.

• Promote express to a runtime dependency and add a 'start' script, since HTTP-server mode needs express at runtime.
• Add .dockerignore, docker-compose.yml (with an optional Postgres profile) and docs/docker.md.
• Postgres stays optional: without POSTGRES_URL the app renders cards with no database.
• Structure leaves room for a future static-SVG generation mode.

Closes #287

[vercel]

@joaoclaudio82 is attempting to deploy a commit to the martin-mfg's projects Team on Vercel.

A member of the Team first needs to authorize it.

[joaoclaudio82]

This PR addresses #287 by adding a container image that runs the backend as an HTTP server, so the cards can be served outside Vercel.

A few notes for reviewers:

• The Postgres dependency is optional. Without POSTGRES_URL the app renders cards with no database, so the image works standalone with just a PAT_* token.
• express was moved from devDependencies to dependencies since HTTP-server mode needs it at runtime, and the lockfile was regenerated to match.
• The build stages are structured so a future static-SVG generation mode can be added as an alternative command without reworking them.

I've run docker build and the container locally against a token and confirmed the /api routes respond. Happy to adjust anything the review turns up. The failing Vercel checks look like the standard fork authorization gate rather than a code issue, but let me know if I've missed something.