Add await db.refresh() calls with relationship attributes before accessing
test_job.executions and execution.backups in async test context.
Fixes lazy loading errors in:
- test_execute_full_backup
- test_execute_incremental_without_full
- BackupEngine class executes jobs and creates backups
- Automatic fallback from incremental to full if no parent exists
- SHA-256 checksum calculation for integrity
- Tests for full backup and incremental fallback