Implement DatabaseKeyStore Backend

Create a database-backed storage system for SSH private keys that encrypts keys before storing them. Implement automatic cleanup of expired keys and proper database integration for production environments.

The issue names no files, tests, or entry points. Start by finding the existing SSH private-key storage interface and database integration, then determine how encryption and expiration are handled elsewhere. Done means a production-ready database backend stores encrypted keys and automatically removes expired keys.

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