feat(testing): configure JUnit XML reporting for all test runners (story 1.1)

Summary

• Configure JUnit XML reporting for all 5 test runners (Jest x3, pytest, Flutter, Playwright)
• Add jest-junit v17 to backend, frontend, and document-repository
• Configure pytest --junitxml output in pytest.ini + os.makedirs in conftest.py
• Configure Playwright junit reporter in playwright.config.js
• Flutter: dart pub global activate junitreport_maintained:2.0.5 (no compatible pub.dev package for Dart 3.11.5)
• Add reports/ and **/reports/ to .gitignore

Test plan

• Backend: 507 tests pass, reports/jest-backend.xml generated
• Frontend: 174 tests pass, reports/jest-frontend.xml generated
• Doc-repo: 117 tests pass, reports/jest-docrepo.xml generated
• pytest: 278 tests pass, reports/pytest-report.xml generated
• Flutter: 105KB JUnit XML generated via flutter test --machine | tojunit
• Playwright: config syntax validated
• All lint checks pass

Closes #772 (closed)