Add database backup download for admins

[OlivierJM]

Feature Parity: Database Backup Management

Legacy app allows admins to download database backups; sparked-next has no backup functionality.

Legacy capabilities

• Admin can download a database dump as dump.tar.gz
• Restoration capability from downloaded backup
• Accessible from admin dashboard

Acceptance criteria

• GET /api/admin/backup — trigger backup generation and download (admin only)
• Backup includes all collections (users, media, institutions, etc.)
• Download as compressed archive (.tar.gz or .zip)
• Backup button on admin dashboard
• Consider S3 backup storage for serverless environments (primary storage is S3 already)

Identified via feature-parity audit against legacy Sparked (Meteor) app.