Fix CI: add linting with pip install ruff

.gitea/workflows/ci.yml

@@ -14,8 +14,16 @@ jobs:
       - uses: actions/setup-python@v5
         with:
           python-version: '3.11'
-      - name: Install and test
+      - name: Install dependencies
         run: |
           cd app
           pip install -e ".[dev]"
+      - name: Run tests
+        run: |
+          cd app
           pytest tests/ -v
+      - name: Run linting
+        run: |
+          cd app
+          pip install ruff
+          ruff check .