fix: update Go version to 1.26.1 in multiple workflow files for consistency and security improvements

.github/workflows/codecov-upload.yml

@@ -47,7 +47,8 @@ jobs:
       - name: Set up Go
         uses: actions/setup-go@4b73464bb391d4059bd26b0524d20df3927bd417 # v6
         with:
-          go-version: ${{ env.GO_VERSION }}
+          go-version: "1.26.1"
+
           cache-dependency-path: backend/go.sum
 
       # SECURITY: Keep pull_request (not pull_request_target) for secret-bearing backend tests.