feat: add storage tiering automation

[prismn]

Summary

• add Terraform lifecycle rules for the backup bucket to transition data through cheaper storage classes over time
• add an access-analysis script that classifies S3 objects by recency and request volume
• add documentation for the tiering workflow, retrieval commands, and cost review process

Validation

• bash -n infrastructure/scripts/analyze-access.sh
• bash infrastructure/scripts/analyze-access.sh --bucket gistpin-backups --output /tmp/storage-tiering.json
• npm run build (Backend)
• git diff --check

Closes #583