Add ML training pipeline with hyperparameter tuning and cross-validation

Machine Learning Training Pipeline

Adds comprehensive ML training jobs:

Long-Running Jobs:

• Hyperparameter tuning (Optuna): 300s
• Model training with 5-fold CV: 250s
• Feature engineering pipeline: 180s
• A/B test analysis: 150s
• Model deployment validation: 120s

Total pipeline time: ~17 minutes

Demonstrates ML-specific CI/CD bottlenecks for knowledge graph analysis.