Replace the stub report_job_completion gRPC handler with a real
implementation that:
- Validates job_id as UUID upfront
- On failure: updates DB status to Failed (best-effort), returns "failed"
- On success: updates DB status to Completed, looks up model_type and
symbol from child_jobs table, registers with PromotionManager, and
returns the actual promotion status string
Also adds JobSpawner::get_job_by_id() to fetch individual child jobs
from PostgreSQL, and two new tests covering promotion status mapping
and the PromotionManager integration path.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
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