ci(tests): update PR workflow for backend tests

- Add cache-dependency-path for npm caching in GitHub Actions.
- Set working directory for dependency installation and test execution to apps/backend.

Author: markmdev

.github/workflows/pr-tests.yml

@@ -22,12 +22,15 @@ jobs:
         with:
           node-version: 20
           cache: "npm"
+          cache-dependency-path: apps/backend/package-lock.json
 
       - name: Install deps
         run: npm ci
+        working-directory: apps/backend
 
       - name: Run tests
         run: npm test -- --ci
+        working-directory: apps/backend
 
   python-tests:
     name: Python tests (lemmas)