feat(m14): backup & admin (zip via fflate, admin guard, tests, e2e)
- BackupService (admin-only: createBackup zips DB+uploads via fflate to
backups/backup-<ts>.zip, listBackups newest-first, getBackupPath with
path-traversal guard)
- Admin backup APIs: GET/POST /api/admin/backups, GET /api/admin/backups/:filename
- Admin backups screen (admin-only) + BackupCreateButton
- Playwright globalSetup seeds a system_admin user for admin E2E
- add fflate dependency
- Unit tests (BackupService: zip content, list, admin guard, traversal) +
e2e backup (create/list/download + non-admin 403)
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