Add auto-formatting GitHub workflow

This commit introduces a new auto-format GitHub workflow. This is designed to automatically format code for pull requests and pushes to the 'master' branch, using self-hosted runners with a 30-minute timeout. The code formatting tool used is ruff.

.github/workflows/auto-format.yml

@@ -0,0 +1,43 @@
+name: Format code
+on:
+  pull_request:
+  push:
+    branches:
+      - 'master'
+
+concurrency:
+  group: ${{ github.workflow }}-${{ github.ref_name }}
+  cancel-in-progress: true
+
+jobs:
+  format_backend:
+    name: Format with ruff
+    runs-on: [ self-hosted, small ]
+    timeout-minutes: 30
+    steps:
+      - uses: actions/checkout@v4
+        with:
+          repository: ${{ github.event.pull_request.head.repo.full_name }}
+          ref: ${{ github.event.pull_request.head.ref }}
+
+      - name: Install Poetry
+        run: pipx install poetry==$(head -n 1 .poetry-version)
+
+      - uses: actions/setup-python@v5
+        with:
+          python-version-file: '.python-version'
+          cache: 'poetry'
+
+      - name: Install dependencies
+        run: poetry install --only main,dev --no-interaction --no-ansi
+
+      - run: poetry run ruff --fix --unsafe-fixes --preview --exit-zero .
+      - run: poetry run ruff format .
+
+      - name: Commit changes
+        uses: EndBug/add-and-commit@v9
+        with:
+          fetch: false
+          default_author: github_actions
+          message: 'Backend: Auto format'
+          add: '.'