feat: add admin UI for export/import data migration

- Admin panel page at #Admin/dataMigration with two-column layout
- Export: filter by case status, skip attachments option, progress + log
- Import: select backup, preserve IDs / skip duplicates, dry run support
- List/delete existing backups with full manifest details
- 5 API endpoints (status, export, import, listBackups, deleteBackup)
- BufferedIO adapter to capture CLI output for API responses
- Admin-only access on all endpoints
- i18n: English + Hebrew (fa_IR) translations

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

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