fix: update GitHub Actions to use correct go.sum path

- Add cache-dependency-path: backend/go.sum for Go setup action
- Fixes cache restore failures in CI/CD pipeline

.github/workflows/ci-cd.yml

@@ -37,6 +37,8 @@ jobs:
       uses: actions/setup-go@v4
       with:
         go-version: '1.24'
+        cache: true
+        cache-dependency-path: backend/go.sum
 
     - name: Install backend dependencies
       run: |
@@ -91,6 +93,8 @@ jobs:
       uses: actions/setup-go@v4
       with:
         go-version: '1.24'
+        cache: true
+        cache-dependency-path: backend/go.sum
 
     - name: Run go vet
       run: |