dify - 💡(How to fix) Fix [Refactor/Chore] Optimize free plan workflow run cleanup batching [1 pull requests]

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Description

The free plan expired workflow run cleanup currently scans expired workflow runs by time range, loads full WorkflowRun ORM objects, checks the billing plan for tenants found in each batch, and then deletes only runs owned by eligible free-plan tenants.

I would like to refactor this flow so the cleanup job has an explicit lightweight pipeline:

1. Fetch candidate workflow run cleanup references only, such as id, tenant_id, and created_at, instead of full WorkflowRun rows.
2. Extract candidate tenant IDs and use billing information to determine eligible free-plan tenants, preserving the existing sandbox plan, grace period, and cleanup whitelist behavior.
3. Push tenant_ids=eligible_free_tenants down into the repository query so the target batch only includes free-plan tenants.
4. Add repository methods for cleanup-specific lightweight refs, for example WorkflowRunCleanupRef(id, tenant_id, created_at), and count/delete related records by workflow run IDs.
5. Keep deletion and dry-run counting based on run IDs rather than full WorkflowRun models.

Motivation

This should reduce memory and database load for large cleanup windows, especially when many scanned workflow runs belong to paid or otherwise ineligible tenants. It also makes the cleanup logic easier to reason about by separating candidate discovery, billing eligibility, and repository-level target selection.

Additional Context

The intended scope is the backend retention cleanup around clear_free_plan_expired_workflow_run_logs.py and the API workflow run repository implementation. The behavior of the existing CLI options, retention windows, dry-run output, metrics, billing grace period, and whitelist checks should stay the same.