Add staging S3 cleanup for Active Storage orphans after DB restores

[leonardow-unep-wcmc]

Summary

• Add a staging-only rake task and service to remove orphaned Active Storage objects from the staging S3 bucket after a production DB restore.
• Use the current active_storage_blobs table as the source of truth and keep a restore cutoff based on the latest blob timestamp so newly replicated objects are preserved.
• Log each dry-run candidate key so operators can review the exact deletion set before running for real.
• Include coverage for the cleanup logic and the staging-only guardrail.

Testing

• Added service specs for bucket pagination, dry-run output, cutoff handling, and environment restrictions.
• Existing serializer and export specs were updated to keep coverage aligned with the current Active Storage and taxon concept changes.