Backend:
- GET /admin/memory now accepts user_id query param to filter facts
  for a specific user instead of returning the global view.

Frontend:
- Users table now has "Sessions" and "Memory" buttons per row.
- Clicking "Sessions" switches to Sessions tab and pre-fills search
  with the user id.
- Clicking "Memory" switches to Memory tab and passes user_id to the
  API, showing only that user's facts.
- Manual tab clicks reset filters (search / userId) so the view
  returns to unfiltered state.

Docs:
- docs/api.md updated with user_id param for GET /admin/memory.

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