Add legacy data migration script and docs ...

- scripts/assign_legacy_sessions.py assigns user_id=NULL sessions,
  memory_facts, and memory_summary to a target user
  - Handles UNIQUE constraint conflicts in memory_facts by merging
  - Merges memory_summary if user already has one
- Update docs/auth.md with legacy migration section

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>