- Add BrokerCredential to domain models and repositories
- Implement upsert_broker_credential() for credential storage and rotation
- Implement list_broker_credentials() for credential retrieval
- Implement delete_broker_credential() for credential removal
- Add broker_credentials table schema with (user_id, broker) composite key
- Implement credential version tracking and last_rotated_at timestamps
- Add FakeRepo support for broker credential CRUD operations
- Add comprehensive test coverage for broker credential lifecycle