feat(ml): train ensemble model and generate benchmark report

Results:
  - XGBoost (Optuna 100 trials): AUC=0.7856, Precision@3=0.5783
  - LightGBM (Optuna 100 trials): AUC=0.7833, Precision@3=0.5736
  - MLP (3 layers 256-128-64): AUC=0.7743, Precision@3=0.5643
  - Ensemble (weighted voting): AUC=0.7840, Precision@3=0.5814

  Baseline XGBoost: Precision@3=0.5287
  Delta: +0.0527 (+5.3%) — DEPLOY threshold met (+5%)
  Latency: 35ms/race, 69ms/full-day (well under 200ms limit)

  SHAP: 31/43 features selected, top features: rang_cote,
  implied_prob, cote_direct, ratio_cote_field

  All 12 regression/latency tests passing.

Co-Authored-By: Paperclip <noreply@paperclip.ing>

models/benchmark_report.md

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+# Benchmark ML Ensemble — Turf Prédictions
+
+**Date:** 2026-04-25  
+**Dataset:** 10,899 partants  
+**Holdout:** 2,180 lignes (2026-04-19 → 2026-04-24)
+
+## Résultats
+
+| Modèle | Precision@3 | AUC | Latence/prédiction |
+|--------|-------------|-----|-------------------|
+| XGBoost (baseline) | 0.5287 | 0.7254 | — |
+| xgboost | 0.5783 | 0.7856 | 0.01 ms |
+| lightgbm | 0.5736 | 0.7833 | 0.00 ms |
+| mlp | 0.5643 | 0.7743 | 0.01 ms |
+| **Ensemble** | **0.5814** | **0.7840** | **0.02 ms** |
+
+## Décision de déploiement
+
+- Delta Precision@3 : **+0.0527** (+5.3%)
+- Seuil requis : **+5%**
+- Résultat : **✅ DEPLOIEMENT RECOMMANDE**
+
+## Optimisation Optuna
+
+- Trials XGBoost : 100
+- Trials LightGBM : 100
+- Pruning : MedianPruner
+
+### Meilleurs hyperparamètres XGBoost
+```json
+{
+  "n_estimators": 141,
+  "max_depth": 5,
+  "learning_rate": 0.016298172447266404,
+  "subsample": 0.7660470794373848,
+  "colsample_bytree": 0.471124415020467,
+  "min_child_weight": 14,
+  "reg_alpha": 1.9364166463791586,
+  "reg_lambda": 6.018030083488602,
+  "gamma": 4.614943551368141
+}
+```
+
+### Meilleurs hyperparamètres LightGBM
+```json
+{
+  "n_estimators": 186,
+  "max_depth": 4,
+  "learning_rate": 0.012915117465216954,
+  "num_leaves": 141,
+  "subsample": 0.6193119116922561,
+  "colsample_bytree": 0.539310022549326,
+  "min_child_samples": 9,
+  "reg_alpha": 0.6864583098112754,
+  "reg_lambda": 0.0549259590914184
+}
+```
+
+## Features
+
+- Total features : 43
+- Retenues par SHAP : 31
+
+## Poids de l'ensemble
+
+- xgboost : 0.2316
+- lightgbm : 0.2342
+- mlp : 0.2129