chore: Finalize CI/CD pipeline - add concurrency, test reports, and documentation

- Add concurrency control to workflow (cancel in-progress runs)
- Add JUnit test results artifact upload to backend job
- Add CI status badge to README
- Create comprehensive CI_CD_SETUP.md with branch protection guide
- Create CI_CD_CHECKLIST.md with production configuration details
- Document auto-merge workflow and troubleshooting tips

CI/CD pipeline is now production-ready with:
✅ Frontend: Lint (ESLint 9, --quiet mode) + Build (Vite)
✅ Backend: Compile + Test (JUnit5) + Coverage (JaCoCo)
✅ Auto-merge enabled with branch protection
✅ All artifacts configured (7-day retention)

Author: xukki241

.github/workflows/ci.yml

@@ -6,6 +6,11 @@ on:
 	pull_request:
 		branches: [main, dev]
 
+# Cancel redundant runs when a new push is made to the same branch
+concurrency:
+	group: ${{ github.workflow }}-${{ github.ref }}
+	cancel-in-progress: true
+
 jobs:
 	frontend:
 		name: Frontend (Lint & Build)
@@ -102,3 +107,11 @@ jobs:
 					name: jacoco-report
 					path: backend/target/site/jacoco
 					retention-days: 7
+
+			- name: Upload JUnit test results
+				if: always()
+				uses: actions/upload-artifact@v4
+				with:
+					name: junit-test-results
+					path: backend/target/surefire-reports
+					retention-days: 7