The pipeline migration (20260213) added stageId columns while keeping
the original roundId columns as nullable. The rename migration tried
to RENAME stageId TO roundId but failed because roundId already existed.
Fix: DROP the old roundId column first, then rename stageId → roundId.
Also restore FK constraints on ConflictOfInterest and TaggingJob.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>