feat: require protected environment approval on terraform-destroy

Bind the job to the 'destroy-production' environment so GitLab blocks
execution until an authorized user approves it in the UI, in addition
to the existing when:manual gate.

Configure in GitLab: Settings > CI/CD > Protected Environments
  environment: destroy-production, required approvals: 1

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>