Fix database backup permissions in GitHub Actions

[0xroylee]

Summary

• Preserve the copied PostgreSQL data directory permissions after fs.cp so the backup can be reopened on Linux.
• Remove stale postmaster.pid and postmaster.opts files from copied backups.
• Add coverage for the backup script to verify the copied backup can be reopened and the runtime files stay out.

Testing

• bun run --filter devos-db check
• bun run --filter devos-db typecheck
• bun run --filter devos-db test
• bun run check
• bun run typecheck
• bun test
• Verified the fix in a Linux container reproduction of the failing database test lane