Summary:

• Add a shared ARQ enqueue helper that removes stale completed-result keys for stable ingestion job IDs before enqueueing.
• Use the helper for standard ingestion, update-file ingestion, v2 ingestion, and admin requeue paths.
• Add unit coverage for stale-result clearing, duplicate preservation, delete failure handling, and no-job-id behavior.

Tests:

• uv run pytest core/tests/unit/test_arq_jobs.py -q
• uv run python -m py_compile core/utils/arq_jobs.py core/services/ingestion_service.py core/services/v2_document_service.py core/routes/ingest.py