Add GCS bucket version audit and restore scripts

[anth-volk]

Summary

• Adds gcp/gcs_version_audit.py — a read-only script that lists all object versions in a GCS bucket and classifies what action would be needed to revert each file to its state at a target date
• Adds gcp/gcs_restore.py — a restore script that reverts objects to a target date, with dry-run by default and interactive confirmation before making changes

Both scripts require google-cloud-storage and GCP credentials. Object versioning must be enabled on the target bucket.

Closes #30

Test plan

• Run gcs_version_audit.py with --limit=5 against a test bucket to verify version listing
• Run gcs_restore.py without --execute to verify dry-run output
• Run gcs_restore.py --execute against a test bucket and confirm objects are restored

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