ci: Pass ANTHROPIC_API_KEY to backend tests

Enables integration tests requiring real API key to run in CI.

Changes:
- Add ANTHROPIC_API_KEY environment variable to backend-tests job
- Secret is passed from GitHub secrets to pytest process
- Tests in test_worker_agent_integration.py will now run instead of skip

Previously, 6 integration tests were skipped due to missing API key.
Now they will execute and validate real API behavior.

Author: frankbria

.github/workflows/test.yml

@@ -101,6 +101,8 @@ jobs:
           git config --global user.email "actions@github.com"
 
       - name: Run pytest with coverage
+        env:
+          ANTHROPIC_API_KEY: ${{ secrets.ANTHROPIC_API_KEY }}
         run: |
           uv run pytest tests/ \
             --ignore=tests/e2e \