Remove SQLAlchemy ORM relationships from Row classes

Remove SQLAlchemy ORM relationship() definitions from Row classes in the manager models.

## Background

- Row usage has been isolated behind the repository/db_source layers, and the ops pattern (Querier/Creator/Updater/Purger) fetches related data with explicit queries, so ORM relationships are no longer needed.
- Some test cases fail solely because of relationship definitions (mapper configuration/reference issues), so removing them also stabilizes the test suite.

## Scope

- Inventory all relationship() definitions in Row classes and their remaining usages.
- Replace remaining usages with explicit queries via the repository/ops layer.
- Remove the relationship() definitions and related loader options (selectinload/joinedload) that depend on them.

JIRA Issue: BA-7222

Start by inventorying relationship() definitions in the manager model Row classes and tracing their remaining usages through the repository/db_source layers and Querier/Creator/Updater/Purger ops. Check the related tests and loader options such as selectinload and joinedload. Done means remaining accesses use explicit repository or ops queries, relationship definitions and dependent loaders are removed, and the affected tests pass.

Written by the indexing model from the issue text.