docs: capture pg17 staging rehearsal

[luandro]

Summary

• document the Postgres 17 staging rehearsal flow and config-sync helper
• add scripts/copy_config.py to safely replicate Heroku config between staging apps
• record the current pg17 staging environment snapshot and verification commands

Testing

• heroku run --app terrastories-staging-pg17 bin/rails db:migrate
• heroku run --app terrastories-staging-pg17 bin/rails runner "puts ActiveStorage::Blob.service.bucket.name"