Store CSV to S3 for Audit Trail

Generate and store CSV export of transformed journal entries to S3 after successful import.

Note: - Generate CSV after transformation, before or after push to Costpoint - CSV format: line-item level with all columns from PoC (Costpoint codes, amounts, QB audit info) - Store in S3 bucket with organized structure: {customer-id}/quickbooks-imports/{yyyy}/{mm}/{import-id}.csv - File naming: include timestamp and import ID for uniqueness - Set appropriate S3 permissions and lifecycle policies - Store S3 key/URL reference in import history record (TBD-11) - Allow downloading CSV from import history UI - CSV serves as audit trail and backup for troubleshooting - Consider generating CSV even for failed imports (for debugging)

Open questions: - Should we store CSV before or after pushing to Costpoint? - What S3 bucket and region should be used? - What lifecycle policy for old CSV files? (archive, delete after X months)