Stream PostgreSQL backups safely to S3

[frnandu]

Summary

• stream pg_dump through gzip and GPG directly to S3
• upload under an .in-progress key and promote it only after the stream succeeds
• clean up temporary objects when streaming or promotion fails
• ignore .in-progress objects in the S3 backup checker
• use loopback pinentry for noninteractive encryption

Testing

• validated shell syntax for both scripts
• exercised single-database and all-databases streaming with mocked PostgreSQL, GPG, and S3 commands
• verified failed streams and failed promotions clean up the temporary object
• verified the checker query excludes newer .in-progress objects