docs: comprehensive README + API reference + architecture guide ...

- Rewrite README.md with full feature list, data model, deployment,
  security, troubleshooting
- Add docs/API.md with JSON schemas, request/response examples,
  and 3 real-world usage scenarios
- Add docs/ARCHITECTURE.md with component diagram, data flows,
  DB index rationale, defense-in-depth security, scaling notes

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