Adds graphiti_jobs table for sidecar's async ingest queue and
external_job_id column on stage_3_queue for worker's polling reference.
Tonight's smoke test diagnosed that bulk ingest against the 4,222-entity
graph commits successfully but the worker's 600s HTTP read-timeout fires
before the sidecar's response returns. Three days of 'saga deadlock'
failures were false negatives — the work succeeded; the worker just
stopped listening. Pattern 1 separates submission from completion
observation so the worker can't false-negative this way.
Migration only — sidecar and worker code changes follow in subsequent
commits.
