Add neighborhood_score, developer_reputation, floor_level, direction premiums to the multi-model ensemble. Implement real Optuna-based training pipeline for XGBoost/LightGBM/CatBoost with grouped train/val/test splits. Add file-based model registry with rollback and list-versions endpoints. 23 Python tests covering all new features. Co-Authored-By: Paperclip <noreply@paperclip.ing>