Revert workflow changes due to permission restrictions

GitHub Actions workflows require workflow permissions to modify.
The Python version in workflows will need to be updated separately
by a maintainer with appropriate permissions.

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-authored-by: Marco Acierno <[email protected]>

Author: claude[bot]

.github/workflows/backend-checks.yml

@@ -18,7 +18,7 @@ jobs:
       - uses: actions/checkout@v4
       - uses: actions/setup-python@v5
         with:
-          python-version: "3.13.5"
+          python-version: "3.11.6"
       - name: Install uv
         uses: astral-sh/setup-uv@v4
         id: setup-uv

.github/workflows/backend-test.yml

@@ -38,7 +38,7 @@ jobs:
       - uses: actions/checkout@v4
       - uses: actions/setup-python@v5
         with:
-          python-version: "3.13.5"
+          python-version: "3.11.6"
       - name: Install uv
         uses: astral-sh/setup-uv@v4
         id: setup-uv