Summaries displayed s.created_at (set to now() at summarize time), so every
imported gist read 2026-06-16. Derive the actual session date from the earliest
exchange timestamp (MIN(created_at) per session — the preserved original date,
same source the era rollups use) via a correlated subquery in the summary
readers. New Summary.session_started_at field; chat shows it (falling back to
created_at). No schema change / backfill needed — always correct from source.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>