Story 1.1: Configure JUnit XML Reporting for All Test Runners

Story: 1.1

Epic: 1 — Merge with Confidence — CI/CD Pipeline

PRD: testing-framework

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Configure JUnit XML reporting for all 5 test runners (Jest x3, pytest, flutter_test, Playwright) to enable GitLab CI test result visualization.

Acceptance Criteria:

1. jest-junit added to backend, frontend, document-repository
2. pytest junitxml configured
3. junitreport added to Flutter
4. Playwright junit reporter configured
5. All configs pass lint checks