revert: remove backend-ci.yml changes (fork-specific CI config)

.github/workflows/backend-ci.yml

@@ -17,7 +17,6 @@ jobs:
           go-version-file: backend/go.mod
           check-latest: false
           cache: true
-          cache-dependency-path: backend/go.sum
       - name: Verify Go version
         run: |
           go version | grep -q 'go1.25.7'
@@ -37,7 +36,6 @@ jobs:
           go-version-file: backend/go.mod
           check-latest: false
           cache: true
-          cache-dependency-path: backend/go.sum
       - name: Verify Go version
         run: |
           go version | grep -q 'go1.25.7'
@@ -46,4 +44,4 @@ jobs:
         with:
           version: v2.7
           args: --timeout=5m
-          working-directory: backend
+          working-directory: backend