🚨(ci) limit print check to backend

We added a feature to print documents directly
from the browser. The function is called
`window.print()`, this name collides with Python's
`print()` function. To avoid false positives in our
CI when checking for print statements, we limit
the search to only the backend code.

.github/workflows/impress.yml

@@ -27,7 +27,7 @@ jobs:
       - name: Enforce absence of print statements in code
         if: always()
         run: |
-          ! git diff origin/${{ github.event.pull_request.base.ref }}..HEAD -- . ':(exclude)**/impress.yml' | grep "print("
+          ! git diff origin/${{ github.event.pull_request.base.ref }}..HEAD -- src/backend ':(exclude)**/impress.yml' | grep "print("
       - name: Check absence of fixup commits
         if: always()
         run: |