Get workspace version

Determines whether the workspace should use Opik V1 (legacy workspace-scoped) or Opik V2 (project-first) navigation. The backend is the single authority for this determination, clients must never derive the version themselves. Determination logic (priority order): 1) Feature flag override (TOGGLE_FORCE_WORKSPACE_VERSION) 2) Auth one-way V2 gate (authenticated mode only, not yet implemented) 3) Version 1 entity check (entities without project_id) 4) Fallback on failure In unauthenticated mode (authentication.enabled=false), auth steps are skipped. Called by the frontend on workspace load.