fix: add backend flag to Codecov coverage upload

Without flags, Codecov couldn't distinguish between backend and
frontend coverage reports. Now both have separate flags:
- backend: Python tests coverage
- frontend: TypeScript tests coverage

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: MarkusNeusinger

.github/workflows/ci-tests.yml

@@ -134,6 +134,7 @@ jobs:
       with:
         token: ${{ secrets.CODECOV_TOKEN }}
         files: ./coverage.xml
+        flags: backend
         fail_ci_if_error: false
 
     - name: Skip notice