docs: add Console auth evaluation plan and update roadmap with auth milestones

- Create CONSOLE_AUTH_PLAN.md: detailed evaluation of @objectstack/client + better-auth
  for building complete login-to-usage Console flow
- Update ROADMAP.md: add Authentication & Console Gaps (P0) section,
  add Q1 §1.5 Console Authentication System (6 weeks, 3 phases),
  update Q2 §2.3 System Objects to reference Q1 auth work

Co-authored-by: hotlong <50353452+hotlong@users.noreply.github.com>

Author: Copilot