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>

Document multi-user auth system ...

- New docs/auth.md — comprehensive auth guide: OAuth flow, roles/permissions,
  webhooks, env vars, setup instructions, API reference, WebSocket auth,
  testing with mocked auth
- Update docs/config.md — add gnexus-auth OAuth env vars table
- Update docs/api.md — add /auth/*, /admin/*, /webhooks/gnexus-auth endpoints
- Update docs/index.md — add auth.md to file map
- Update docs/sessions.md — document user_id column and list_all filtering
- Update docs/memory.md — document user_id scoping in facts and summaries

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