docs: update documentation to match refactored codebase
- Delete obsolete plans/code-review.md (all suggestions implemented)
- Rewrite docs/API.md to reflect modular architecture, DashMap, corrected
AppError variants, accurate function signatures and module locations
- Rewrite docs/ARCHITECTURE.md to reflect DashMap concurrency, remove
completed future enhancements, fix AnalysisContext description
- Update docs/USER_GUIDE.md with global CLI path options (--my-cars,
--cars-db, --times-dir, --tracks-db) and updated troubleshooting