Add UNIQUE constraint to FeedbackRow to enforce one feedback per user per run,
enabling upsert behavior in Task 2. Update tests to use distinct user_ids for
multiple feedback records per run, and pass user_id=None to list_by_run for
admin-style queries that bypass user isolation.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>