feat(api): add admin backup endpoint

[jack-arturo]

Summary

• add admin-only GET /backup that streams a restore-compatible tar.gz export of FalkorDB and/or Qdrant
• refactor the existing backup script onto shared backup helpers
• let restore_from_backup.py --backup-dir accept downloaded .tar.gz archives
• document the API backup export workflow

API / config

• GET /backup requires X-Admin-Token or X-Admin-Api-Key using ADMIN_API_TOKEN
• regular AUTOMEM_API_TOKEN auth is intentionally bypassed for this endpoint
• optional include=falkordb,qdrant defaults to both stores

Breaking Changes

None.

Testing

• PYTEST_DISABLE_PLUGIN_AUTOLOAD=1 .venv/bin/pytest -q
• PYTEST_DISABLE_PLUGIN_AUTOLOAD=1 .venv/bin/pytest tests/test_backup_endpoint.py tests/contracts/test_routes_contract.py -q
• pre-commit hooks on commit

Railway

Ready for branch deployment smoke testing on Railway.