Fix MySQL backups with generated search columns

[zhenghaoz]

Backups produced by the MariaDB client against MySQL include explicit values for the generated items.gorse_search_document column, causing restores to fail with ERROR 3105. Use the official MySQL 8.4 client in the backup image and export INSERTs with explicit column lists so generated columns are omitted and recomputed during restore.

The backup script also uses a consistent transaction snapshot, disables column statistics and GTID output for portability, rejects MariaDB dump clients, and enables pipefail so failed exports cannot be uploaded as successful backups.

Validation:

• Built the updated backup Docker image and checked MySQL, s3cmd, and gzip availability.
• Used the new container to dump and restore a test table with a stored generated column and FULLTEXT index on MySQL 8.0.32; verified restored generated values and removed the test table.
• Verified MariaDB clients and failed dumps stop before S3 upload using command stubs.
• Passed bash -n scripts/backup.sh and git diff --check.

Rebuild the backup service to apply: docker compose up -d --build backup. Existing backups are unchanged and may still require repair.