Add JobSpawner as the highest-priority dispatch path in start_training.
When job_spawner is available, training requests are persisted to
PostgreSQL via spawn_batch() and a batch ID is returned immediately.
The queue consumer (Task 3) will later poll for pending jobs and
dispatch them to K8s.
Priority order: JobSpawner (DB) → K8sDispatcher (direct) → orchestrator (in-process).
Also adds test_start_training_model_binary_mapping unit test.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
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