ci: fix go module cache key to use backend/go.sum

.github/workflows/ci.yml

@@ -36,7 +36,9 @@ jobs:
         uses: actions/cache@v4
         with:
           path: ~/go/pkg/mod
-          key: ${{ runner.os }}-go-${{ hashFiles('**/go.sum') }}
+          # Use the backend module's go.sum for cache key so the cache
+          # restores correctly when modules change.
+          key: ${{ runner.os }}-go-${{ hashFiles('backend/go.sum') }}
           restore-keys: |
             ${{ runner.os }}-go-
       - name: Run tests