Wire JobSpawner into the gRPC service struct so that training jobs can
be persisted to PostgreSQL before being dispatched to K8s. This is the
first step toward a durable job queue that survives pod restarts.
- Add `job_spawner: Option<Arc<JobSpawner>>` to MLTrainingServiceImpl
- Extend `new()` constructor to accept the spawner parameter
- Add `DatabaseManager::pg_pool()` accessor for cheap PgPool cloning
- Construct JobSpawner in main.rs and pass `Some(job_spawner)` to service
- Add compile-time test verifying the field exists
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
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