Add recommendations.Engine.ActiveEmbeddingModel, which reads the
embedding lock from server_settings and returns the locked model (or ""
when no lock is established). Define the catalog
CatalogSemanticModelProvider interface so a later task can scope vector
coverage checks to the active model; the recommendations Engine already
satisfies it. No wiring yet (Task 4 injects recEngine as this provider).
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
