Row-level measurement classifier: per-measurement censorship scoring (Niaki KDD23 inspired)

New POST /v1/measurement/classify scores a single OONI, CensoredPlanet, IODA, or Voidly measurement and returns a probability + SHAP top-5 explanation. Inspired by Niaki et al. KDD 2023. Honest framing: the model learns to reconstruct the labeling rule, so the high AUC is real but reflects label leakage from raw signals, not novel detection ability.