stage2_worker: ON CONFLICT clause resets all run-state fields on re-enqueue

Bug: when a row in stage_3_queue gets re-enqueued (same source ingested
again after Stage 2 re-runs), the ON CONFLICT (source) DO UPDATE clause
updated content fields and reset enqueued_at, completed_at, failed_at,
attempts — but did not reset started_at, failure_reason, or
external_job_id.

Stale started_at from a prior attempt makes the row invisible to the
Stage 3 worker's claim filter (which uses started_at IS NULL). The row
sits queued forever; Stage 3 never picks it up; the source effectively
fails silently after a re-trigger.

Discovered tonight while testing the bulk pathway after the substrate
fix: a journal entry that had been ingested earlier (and manually marked
completed during recovery from a worker timeout) showed enqueued_at
from the new touch but started_at from the original 01:40 attempt. Fix
extends the upsert clause to NULL all run-state fields so re-enqueue
behaves as 'fresh attempt.'

After fix, re-triggered journal entry routed cleanly through Stage 2 →
Stage 3 → bulk pathway → sidecar bulk job → 60ms commit (worst-case
dedup against already-known content).

scripts/stage2_worker.py

@@ -220,8 +220,15 @@ def enqueue_stage3(pg, source, full_text, orientation, metadata,
             supersedes_prior_state = EXCLUDED.supersedes_prior_state,
             state_type_rationale = EXCLUDED.state_type_rationale,
             enqueued_at = NOW(),
+            -- Reset all run-state fields on re-enqueue. Without this,
+            -- stale started_at from a prior attempt makes the row
+            -- invisible to the Stage 3 worker's claim filter (which
+            -- typically uses started_at IS NULL).
+            started_at = NULL,
             completed_at = NULL,
             failed_at = NULL,
+            failure_reason = NULL,
+            external_job_id = NULL,
             attempts = 0
     """, (source, full_text, orientation, json.dumps(metadata),
           state_type, state_type_confidence, supersedes_prior_state,