chore(oss): prepare repository for contributors

[rahuljangirworks]

Summary

Prepare JobSync for external open-source contributors and safer self-hosted setup.

Changes

• Add a security policy, issue templates, PR template, and Dependabot configuration.
• Add the missing Vitest jsdom dependency and a reusable TypeScript check script.
• Run type checking in GitHub Actions CI.
• Make local .env.example setup usable with SQLite.
• Replace placeholder Docker secrets with secure generated values persisted in /data.
• Align contributor documentation with Vitest and the current test workflow.

Testing

• npm ci --ignore-scripts --no-audit --no-fund
• npx prisma generate
• npm run lint
• npm run typecheck
• npm test -- --maxWorkers=2 — 212 files, 2,559 tests passed
• npm run build — passed; existing non-blocking unpdf sourcemap warning
• docker compose config --quiet
• sh -n docker-entrypoint.sh