Index orchestra_workflow_version.workflow

Adds a WorkflowVersionStorageSchema declaring an index on the base-table workflow column, registered via the entity handlers key. The version manager's versions() and its garbage collector both run an all-revisions query filtered by workflow, which was an unindexed scan of the growing version table. Extends EngineIndexTest to assert the index is present.

Pre-1.0, so no update hook: applied by hand on the live site.