feat(playground): update backend API and add analysis documentation

- Expand main.py with new API endpoints and features (+1,559 lines)
- Refactor test_all_apis.py for better test structure
- Add comprehensive change analysis documentation
  - CHANGES_ANALYSIS_COMPLETE.md: Full analysis of 209 files
  - COMMIT_MESSAGES.md: Commit message guide

Files changed: 2 files, +1,559 lines

Author: leebeanbin