Replace 2 stub gRPC handlers with real implementations: - SelectAssets: scores instruments via MLFeatureExtractor, ranks with AssetSelector (TopN/Threshold/Quantile modes), persists to DB - GetSelectedAssets: loads latest selection from asset_selections table Add helper methods: score_instrument, store/load_asset_selection Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>