docs: add CI workflow and testing requirements documentation

- Add GitHub Actions CI pipeline with pytest, ruff, mypy
- Expand Tasks.md with implementation tracking and testing criteria
- Update CONTRIBUTING.md with CI requirements
- Add Testing-Requirements.md with coverage thresholds and PR checks

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

CONTRIBUTING.md

@@ -104,10 +104,11 @@ BanGUI/
 # Backend
 cd backend && pytest --cov=app --cov-report=term-missing
 
-# Frontend — run once
-cd frontend && npm test
+# Coverage threshold: 80%. Build fails if coverage drops below.
 ```
 
+The CI pipeline enforces the same 80% minimum coverage threshold.
+
 ---
 
 ## Stack