fix(deploy): add CI/CD workflow, complete railway.json, and finish DEPLOY.md

[macan88]

Problem

The repository lacked a working GitHub Actions CI/CD pipeline, railway.json was missing the variables section required by Railway's env-var injection, and DEPLOY.md was cut off mid-sentence leaving deployers without actionable steps.

Solution

Added .github/workflows/ci.yml that runs bun install → bun test → tsc --noEmit on every PR and additionally calls railway up --detach (guarded by RAILWAY_TOKEN) only on merges to main, ensuring tests always gate deploys. Completed railway.json with the correct health-check path and restart policy so Railway can reliably promote healthy builds. Rewrote DEPLOY.md end-to-end covering CLI install, railway init/link, secret management, custom domain DNS setup, CI token wiring, and a troubleshooting table.

Testing

• Push a feature branch → CI runs tests, deploy step is skipped (if: github.ref == 'refs/heads/main' guard)
• Merge to main with RAILWAY_TOKEN set → full pipeline runs, Railway health check hits /health returning {"ok":true} before traffic is accepted
• curl -f https://<service>.up.railway.app/health returns 200 within the 10 s timeout configured in railway.json

Closes #4