ci: handle missing backend tests safely

.gitea/workflows/cd.yaml

@@ -60,7 +60,13 @@ jobs:
         run: pip-audit -r platform/backend/requirements.txt
         
       - name: Run Backend Quant Engine Tests
-        run: pytest platform/backend/app/analytics/
+        run: |
+          if find platform/backend -type f \( -name 'test_*.py' -o -name '*_test.py' \) | grep -q .; then
+            pytest platform/backend
+          else
+            echo "未找到後端 pytest 測試檔，改以 Python 編譯檢查作為最低安全閘門。"
+            python -m compileall -q platform/backend/app
+          fi
 
       - name: Setup Node.js Environment
         uses: actions/setup-node@v4