Replace 6 stub handlers returning fake JSON with real gRPC proxies:
- training.rs: list_jobs→ListTrainingJobs, start_job→StartTraining, cancel_job→StopTraining
- tune.rs: start_tune→StartTuningJob, get_status/get_best→GetTuningJobStatus, stop_tune→StopTuningJob
Add ml_training proto module and MlTrainingServiceClient factory.
All 24/24 web-gateway REST endpoints now proxy to real gRPC services.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
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