feat(mlflow): swap default tracking URI to sqlite:///mlflow.db

feat(api): make MLflow registry the serving source-of-truth

feat(api): surface registry version in /health

feat(api): wire /predict drift recording + /metrics/drift endpoint

feat(api): rewire /predict through Depends(get_classifier) with hardened error handling

refactor(api): introduce create_app() + lifespan with SENTIMENT_BACKEND dispatch

chore(repo): bootstrap walking skeleton and governance