feat: implement bundled backend admin plane (move SPA to backend/web)

[ramudaderuta]

This PR implements a bundled Admin Plane for the Chronicle Engine, providing a React-based SPA for operators to manage memories, monitor distillation, and trace recall logic.

Key Changes

• Frontend: Added a React + TypeScript SPA in backend/web/ using TanStack Query/Router.
• Backend:
  • Implemented admin-only JSON APIs under /admin/api/*.
  • Added secure admin auth middleware and rate limiting.
  • Integrated tracing for structured logging.
  • Added an audit log for admin mutations.
• Infrastructure: Updated the Dockerfile to a multi-stage build (Node.js frontend + Rust backend).
• Documentation: Updated README.md and deployment docs with Admin Plane instructions.
• Verification: Added comprehensive integration tests in backend/tests/admin_plane.rs.

Features

• Dashboard with principal activity overview.
• Memory browser and management (CRUD).
• Behavioral guidance inspection.
• Recall Lab for side-effect-free recall simulation and tracing.
• Distill job monitoring and artifact browsing.
• Session transcript viewing.
• Governance review and promotion of candidate memories.