CI: run the checkpoint suite in a venv (host Python is PEP 668 externally-managed)

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_015jtxAiu5dAW4WxCNdGkrkJ

.gitea/workflows/checkpoint.yml

@@ -26,10 +26,14 @@ jobs:
           git fetch -q --depth 1 origin "${{ github.sha }}"
           git checkout -q FETCH_HEAD
 
-      - name: Backend deps
+      - name: Python env
         working-directory: repo
-        run: python3 -m pip install --quiet -r backend/requirements.txt
+        run: |
+          set -eu
+          python3 -m venv .venv
+          .venv/bin/pip install --quiet --upgrade pip
+          .venv/bin/pip install --quiet -r backend/requirements.txt
 
       - name: Checkpoint suite
         working-directory: repo
-        run: python3 scripts/run_checkpoint_tests.py --postgres-port 25690 --backend-port 18690
+        run: .venv/bin/python scripts/run_checkpoint_tests.py --postgres-port 25690 --backend-port 18690