Add: CI/CD support and Web Service API

- Added externallib.php with save_submission web service
- Added callback.php for backend result callbacks
- Added db/services.php for service definitions
- Fixed property names in dta_result and dta_result_summary
- Added .gitignore for temp files
- Added CHANGELOG.md documenting v2.1.0 changes
- Plugin tested successfully with CI/CD pipelines